The Gilbert-Holtzmann Equation

What do you want when you grow up?

At ten years of age, Erin Gilbert wrote her answer to that very question in her A5 notebook - the one with the pink paper and cartoon cats making up the border, dated neatly at the top with her name in the right hand corner - handing in the folded page to her teacher. Her mother would later pin that creased piece of paper to the fridge in the family home, telling her daughter that she could achieve anything she wanted in her time on this earth.

That was before the years and years of forced therapy of course. Back when her parents were still proud of their protégé.

Erin Gilbert wrote three things on that small piece of paper, and holding it in her hands thirty-odd later by pure accident, she saw that she and her much younger self still had much in common.

Number one: I want to do science.

Number two: I want to have lots of friends.

Number three: I want to marry my best friend, like mommy and daddy.

It had been three years since a ragtag bunch of women saved New York City from a ghoul apocalypse. In that time, her three comrades had become family. Her relationship with Jillian Holtzmann (nuclear engineer, unstoppable flirt, and all-around mischievous imp) had flourished quickly - Erin couldn't resist that charm and dimpled grin for long - and a deep love existed between the two, even if some would say they were a strange partnering.

They both had traits the other benefitted from. It worked, and that's all Erin needed.

Looking at the list, she smiled a little. Her job as a Ghostbuster was pretty much the pinnacle of 'doing science', in a really cool way. And sure, she didn't have lots of friends, but the quality of the friends she had sure compensated for the quantity.

She hesitated when she reached the last item.

I want to marry my best friend, like mommy and daddy.

Erin would be lying if she said she had never thought about the idea. She would absentmindedly play with her ring finger, wondering how a simple yet elegant silver ring would feel in place of the nakedness. Daydreams about becoming Dr. Erin Gilbert-Holtzmann occasionally floated around her mind, usually when passing by jewellery stores or when she was caught off-guard by a brilliant Holtzmann smile.

She looked up from where she was sat, watching her lover dance to an 80s playlist and lip sync into a screwdriver microphone as she worked on a new piece of technology for the group. 'It's no wonder she's a powerhouse in bed,' Erin thought with a self-satisfied smirk, seeing Holtzmann thrust her hips in time to the beat.

The pink paper was hidden between the pages of a scientific journal she'd found whilst carrying out a much-needed exodus of her bookshelves. She recalled that her mother had sent it to her a while ago, not long after her father passed away; the Gilbert matriarch had downsized from the large family home to be closer to her only daughter in an attempt to reconcile, and mailed over some forgotten items Erin had long since left behind.

A slow day had led her to read the journal out of boredom, not wanting to tear her friends away from their own work. This action, by pure chance, had revealed a relic from her past, which in turn revealed a ton of realisations that she didn't particularly want to deal with.

She always envisaged she would be married before forty, probably to an older man with a similar background and a similarly successful career. Then life happened. Ghostshappened. Bisexuality happened. Jillian Holtzmann happened. Jillian Holtzmann, who was nine years younger than Erin and a professional oddball. Jillian Holtzmann, who Erin couldn't now imagine life without.

Jillian Holtzmann, the personification of the phrase 'free-spirit', who Erin couldn't imagine tying down. Legally, anyway.

It was, therefore, with the feeling that somebody was stood on her chest, that Erin Gilbert resigned herself to the fact that she could love Holtzmann or be married, but never have both. She figured that as long as she had the engineer in her life, she could easily live with not being married.

She kept the paper between the pages of the journal, appearing to simply be working as opposed to daydreaming. Picking up a stubby pencil, she wistfully ticked in the margin of the first two items, resting her pencil next to the third, leaving a mark that would never be followed through. It was juvenile, she knew it, writing 'I wish' next to the last point.

She sighed, only realising it was aloud when Holtzmann turned her music down.

"Are you alright?" The engineer asked, her head cocked to one side, goggles pushed up onto her forehead, and grease marks under her eyes. Her blonde hair, as usual, was organised chaos, piled on top of her head in loose curls. "Do you not have enough oxygen? Or are you unhappy?"

"What? No," Erin denied quickly, hand waving in dismissal of the idea. "No I'm not unhappy, I mean. I have enough oxygen. Probably. What am I saying?"

She was a terrible liar; even worse when she tried to lie to Holtzmann. The engineer gave her that pointed stare, the one with wide eyes and her lips naturally curled into a slight smile, an expression that usually meant 'Erin, I was not born yesterday, I was born on the seventh day of February in nineteen-eighty-four thank you very much, now tell me what is wrong,' which would open the floodgates to the physicist spilling her proverbial emotional guts.

This time was different, however, and Erin knew Holtzmann couldn't be told about this, because she would try her best to make it right, in the way that she always did. But she didn't want the engineer to sacrifice her own happiness for the sake of Erin's.

And there was nothing wrong. She loved Holtzmann, and Holtzmann loved her. It was a superfluous desire, that was all.

So why did it feel like more than that?

"It's just a really old equation I could never figure out," Erin lied, a little more convincingly for once. "I knew there was a reason I hid this book between the rest of them."

She smiled in what she hoped was a reassuring way, internally sighing in relief when Holtzmann shrugged and turned the music back up, shooting her a wink and performing a rude gesture with her tongue in Erin's direction. At least that lie had worked.

"Erin!" Abby called from the floor below, her saving grace. "Kevin deleted our expenses spreadsheet again, and I can't find your back up USB!"

"Patty said we were just keeping our heads above water this month. I didn't want us to drown, so I fixed the problem," she heard Kevin say in his defence, followed by an exasperated 'It's just an expression! Man, I can't believe you,' from the woman in question.

The physicist rolled her eyes, smiling nonetheless, bidding farewell to her girlfriend and making her way downstairs, preoccupied with remembering where she had put the back-up USB and leaving the journal closed and forgotten on the table.

Jillian watched Erin depart from the corner of her eye, setting down her blowtorch when she was out of sight. She ambled over to where Erin had been sat looking adorably concerned and put out. Any physics equation Dr. Gilbert could not solve just had to be good, and Holtzmann would earn bragging rights for at least six months if she could solve a problem her girlfriend couldn't.

"Alright, listen here," she said to the book, picking it up and appraising it suspiciously. "Erin might've gone soft on you, but I'll make you tell me all your filthy secrets."

If the journal could gain sentience and see the manic expression on the engineer's face, it would surely be terrified.

She thumbed through the pages, face scrunched in confusion when very little of the content seemed to actually be equations to be solved. A bright object caught her attention as she flicked her way to the end, contrasting against the slightly yellow and dog-eared pages. Deft fingertips plucked the folded piece of paper from the book, her other hand slamming it shut and dropping it to the table.

'Filthy secrets indeed,' Holtzmann thought, smile widening. She unfolded the paper, seeking its contents.

If her eyebrows weren't attached to her face, they would have shot holes through the ceiling.

And if her heart wasn't contained in her chest, it would have ascended with them.

Jillian Holtzmann was an engineer; she had crafted many inventions that defied belief, gadgets that displayed the unbelievable capabilities of modern science and pushed the boundaries of technology. Hell, she even had the blueprints for an automatic Pringle feeder.

But when she went back to her work table, rolling out a large piece of blue paper, she began to draw out a creation in silver marker unlike any other. She mused that though this plan would not gain her any prestigious scientific accolades, it would serve to be the most important one she'd ever made.

Holtzmann's plan, drawn up after sneaking out of the bed she shared with Erin over the space of three nights, and fuelled by a few sips of cold coffee and pure heart, consisted of a set of precise methodological stages.

Stage one: Tell the friends/accomplices.

"Abby, Patty. My good friends, adoptive mothers, and partners in slime. You're probably wondering why I have gathered you here today, here in this dingy alleyway, as my beloved Erin attends her monthly meeting with the city's good mayor."

"I'd appreciate it if you let us know quickly Holtzy," Patty said, shivering, arms wrapped around herself. "I'm packing bullets here, and I ain't talking 'bout the ones in my gun."

Abby nodded in agreement. "Could we not have done this inside? Was that not an option?"

Ignoring her friends trembling in the cold winter air, and warmed by her own excitement and adrenaline, Holtzmann unrolled her blueprint with a flourish, quickly pinning it to the wall. Double sided sticky tape was a gift.

"Welcome, ladies, to what I call 'The Gilbert-Holtzmann Equation' - hold on tight to your proton fanny packs, because we're solving for x, and x is a frickin marriage proposal."

The other two women stood with mouths agape, the information processing slowly. They looked to each other, confused, looking back to the blueprint, across to Holtzmann's excited grinning face, back to each other.

Abby was the first to attempt to verbalise. "Y-You're going to ask Erin to marry you, that's what you're saying?"

Holtzmann nodded, smile firmly in place, her happiness enough to light up a funeral home.

"Like, the real 'I do, I do, I love you' thing?" Patty asked, voice raising an octave toward the end. "Like, two rings and a Shania Twain playlist kind of thing?"

"God, secretly I love Shania Twain, but she's overplayed at weddings," Abby intercepted, Patty agreeing, the two almost going off on a tangent until the engineer interrupted.

"The. Real. Thing." Holtzmann stated, raising her arms to bring attention to her masterpiece on the wall. She watched her friends and their as-of-yet lack of reaction, her smile fading slightly. She added: "That's if you guys approve."

"Approve?" Abby repeated, almost in a whisper. "Approve?!" She looked to Patty. "Our Holtz and Erin are getting married!"

Both Patty and Abby began to shriek in excitement, bouncing up and down and hugging each other, relieving the nervous tension that had coiled in Holtzmann's stomach.

"So, will you guys help me out?" She asked, her grin returning when they enthusiastically shook their heads to say yes.

Producing three small walkie-talkies from a box next to her, she handed two to her partners in crime.

"Communication is key to this plan," she said sagely, looking them both intently in the eye to ensure they understood this central tenet of the operation. "And we need code names. Please turn over your handset to discover your code name."

"Soup-er trooper?" Abby said, frowning. "I feel like this one works better on paper. I like the intent though, seven out of ten, nice work."

"At least yours sounds like a title, I'm mad as hell," Patty responded.

Abby laughed. "To make you mad it must be pretty bad,"

"No, I'm actually called Mad-As-Hell," the history buff clarified, showing Abby the sticker on the back of the device. "That's one hell of a throwback, Holtzy."

"What can I say, nothing suits you better than a twenty foot tall beast on your shoulders, Patty," Holtzmann quipped, winking. "Now the stickers will self-destruct in five seconds to preserve the confidential nature of this operation."

The other Ghostbusters dropped their devices in panic, springing back.

"Just kidding, wanted to see if they were durable," Holtzmann said, relishing in the annoyed looks shot at her.

"Is your code name Comedian, because it sure as hell should be," Patty mumbled sarcastically, bending to pick up her walkie-talkie.

"Close, but no cigar, Tolan. My code name is Phillian Joltzmann, Phil for short."

"Real subtle."

"Kevin is Kevin, don't want to overload the poor hamster wheel in his brain."

"Good thinking."

"And Erin is the love of my life."

"Doesn't exactly roll off the tongue, but we'll roll with it."

"Oh that's not her code name. We can just call her Erin."

Stage one-point-five: Tell Kevin.

"Kevin, Holtz is planning to propose to Erin."

Cogs turned. Slowly.

"Alright, sweet. I'm getting ice cream, anyone want some?"

Stage two: Reconnaissance.

A day had passed since Holtzmann's revelation, and the next stage was to be triggered. Like any good scientist, Jillian had done her best to account for any extraneous variables - such as the notion that Erin may not in fact be interested in marriage, and the list had been an outdated fluke.

To eliminate this unwanted possibility, long-time friend of the physicist Abby Yates had been assigned to step up to the plate.

"The parameter I've set for success at this stage is five instances of interest in marriage-related items or concepts," Holtzmann explained, pen behind her ear. "We already have one." She indicated to the pink notebook paper pinned to a board along with her plan, forming the decor for an entire wall in Patty's room, which had become their base of operations. This reduced the risk of Erin discovering the entire thing.

"I'd say that's a pretty strong indicator right there," Patty said, shrugging. It was clear that Erin was ridiculously attracted to Holtzmann, and vice versa, their relationship never really losing that honeymoon period. She didn't see the point in stage two of the plan, but if it helped Holtzmann's confidence in getting to the end goal then she couldn't complain much.

Holtzmann pushed a printout of a section of city streets towards Abby, roads highlighted with an orange marker pen. "I've planned a route that takes you past three jewellery stores, five stores with wedding dresses in the window, a bar that offers a twenty-percent discount on wine when catering for hen parties, and a good Chinese takeout place at the end. I figured you'd be hungry."

"And this is why I love you. I'll let you know what happens."

Abby left the room, making a beeline straight for Erin, who turned out to be scrawling equations and notes on her large whiteboard. Half the time, Abby didn't know what the hell her best friend was trying to figure out. She snorted to herself, remembering when she'd said jokingly 'All these sums and you still can't figure out how much you like Holtz'. That day ended with a teary-eyed Erin and Abby eating ice cream in the latter's room as the former confided the extent of her feelings and sexuality to her best friend.

Oh, how far they had come.

"Something funny or you got something stuck up your nose?" Erin asked, not turning away from the whiteboard.

Biting her tongue to refrain from responding with a quip that would give away Holtzmann's plan, Abby ignored the question. "I was thinking about going for a walk. Wanna join me?"

"I'd love to, but I've got to finish up what I'm doing here."

"C'mon Erin," Abby whined. She wasn't proud of her chosen method of persuasion, but it was the best one to ensnare a wild Erin Gilbert. The Guilt Trip. "We've hardly spent any time together, I miss our long talks. YatesBert needs a revival. Please? Just a walk? You can't leave your sums just for that?"

Erin sighed, turning and smiling at her friend, rolling her eyes in jest. "Alright, fine. You always know how to get to me. Let me grab my coat."

The two made their way out of the building, wrapped up against a biting wind that had picked up in the early afternoon. Abby led the way, the memorised map that Holtz had presented to her serving as a GPS in her mind.

"So…" Erin said, drawing out the vowel. "Where are we going?"

"Oh, you know, anywhere. Everywhere. Places between," Abby replied nonchalantly. They were rapidly approaching the first jewellery store on the route. Using this to her advantage, she drew attention to it, stopping and placing her hands against the window as she peered in. "God Erin, look at those rings! You could grate cheese on those diamonds!"

She turned her head to study Erin's reaction, noting the way her smile faltered slightly as she looked at the display.

"Pretty nice, if you're into that," she said, and Abby could sense the sadness in her voice.

"Hey Erin, you okay?" Abby asked, concerned.

Erin sighed heavily. "Just looking at a life I'll never have, no problem, I'm good. Great, even." She half-heartedly aimed finger guns at her best friend, before turning away and continuing to walk ahead.

Abby followed her, knowing Erin would soon elaborate, unable to hold back on her emotions once she had let her composure slip.

"I just… sometimes I wish I could have everything, you know?" Erin divulged, as predicted. Her friend fell into step next to her, hands in her pockets, content to listen to something that had clearly bothered Erin for a while. She didn't expect this sudden sadness.

"I-I know it's just society conditioning us to think it, but I'd love to be married, you know? God, I sound so awful and ungrateful right now. I'm awful. Ignore me."

"Erin, you're not awful, you're the most compassionate and caring person I've known in my life. You're allowed to want things you don't have. Hell won't open beneath us and swallow you up, I promise," Abby reassured. Erin looked at her, prompted to carry on with a nod.

"Holtz is everything I could ever want, and she's everything I never imagined I could have. But she just doesn't… fit… with that stupid idea of marriage I have and can't get rid of, and I know it's ridiculous-"

"It's not ridiculous," Abby consoled, almost squealing at the irony of it all. She was struggling not to put her friend out of her misery - seeing Erin upset was unbearable. Scrapping parts of Holtzmann's plan, she decided to just skip right to the end to the Chinese place. Erin's admission was more than enough to provide wind for the sails. "Come on, I've got wontons with my name on 'em just waiting for me to arrive."

Later, when they returned home to their respective rooms of the firehouse, with Erin in a considerably better mood, Abby fished the walkie-talkie from her pocket.

"Holtz. Holtz?" She called.

"Phillian Joltzmann would like to remind Souper Trouper of the importance of stealth in the art of espionage. Over."

"Oh for God's sake," Abby mumbled, rolling her eyes. "Souper Trouper would like to inform Phil that stage three is good to go. Hurry up and put a ring on it, hot stuff."

A long pause followed, and Abby could practically hear Holtzmann's smile through the line.

"Nice work. Stage three a-go-go. I thank you, and I love you. Over and out."

Stage three: The set-up.

Stage four: Tell the mothership.

These two stages of the plan occurred simultaneously. Patty and Abby would carry out stage three, setting up the environment for the proposal to work the way Holtzmann had envisaged. It was a large task, but one they knew they could handle.

Stage four, Jillian would handle alone. Since this wasn't the 1800s and Erin wasn't property, Holtzmann wasn't asking for Mrs Gilbert's permission for her daughter's hand in marriage. She did, however, know how close the woman was to both Erin and Abby once they had reconciled after saving the city, and though she had only met her once very early on in their relationship, Holtzmann wanted to involve Mrs Gilbert in this chapter of her daughter's life - despite never truly forgiving the woman for putting Erin through all those years of therapy.

Dressed in her favourite necktie, a grey patterned waistcoat with matching pants and her hair as organised as chaos could be, Jillian was ready. Upon questioning from Erin, Abby and Patty had claimed they were heading out for a ghostly stakeout to give themselves time to set up. Kevin was out doing whatever Kevin did, and Holtzmann was hoping she could just sneak out into the Ecto-1.2 (rest in peace, Ecto-1) and avoid Erin's inquisitive stare altogether.

"Oh hey Holtz, where you going? I must have missed the memo for 'Let's Ditch Erin Day'," Erin greeted, emerging from the second floor and catching Holtzmann with her hand on the doorknob.

Cringing at the door in exaggerated frustration before schooling her features into a calm stillness, Holtzmann turned to her girlfriend. Before she could formulate a convincing lie, Erin interrupted after giving her a once-over.

"You look… fancy," Erin said, eyes lingering on Jillian's body. "Not that you don't always look fancy. Just, more fancy. Formal fancy. You're not sneaking out to see another woman, are you?"

The question was asked in jest, but Holtzmann knew deep down Erin was still very insecure in herself, always had been. She smiled one of her rare serious smiles, hoping to reassure the other woman.

"Dr. Gilbert, I could sue you for slander. Why would I sneak out to see another woman when I can view my entire world right here in this building?"

Erin blushed, still easily dismantled by that charm even years on. "Y-You stop that," she finished lamely, unable to engage her brain to say anything more. "So if the ladies of New York are safe from you being… you, then where are you going? I sound like such a mom, I know, I just like knowing you're all safe."

"Craigslist is home to equipment that violates hundreds of health and safety regulations. I may or may not be acquiring said equipment," Holtzmann lied, "I know you like a naughty woman, Erin, so you'll let me go this time."

To Jillian's surprise, Erin took a step into her personal space, straightening her necktie before yanking it, bringing them face to face.

"Well Dr. Holtzmann, I was thinking I would severely reprimand you for your bad behaviour," Erin spoke quietly, eyes focused on Holtzmann's lips. "Since Patty and Abby are out, and we have the place to ourselves…" She trailed off, letting the words sink in and feeling delight when Jillian visibly shuddered, blue eyes darkening with a familiar hunger.

Holtzmann struggled to stay focused, and the hardest thing she had ever done was reject an advance from her girlfriend. It was also the first time she had ever done so - knowing it was for the greater good was her only consolation. On her way out of the door, she missed the hurt look on Erin's face, the other woman wondering exactly what she'd done wrong.

Settling herself in the driver's seat, Holtzmann started the ignition. Her walkie-talkie burst into life from where it was taped to the dashboard.

'Phil, you there?' Patty's voice crackled through.

"Phillian Joltzmann reporting in. Hit me with an update, Mad-As-Hell."

'We're doing good here, everything is almost set up for whenever you're ready to break a leg bending a knee.'

"Fabulous. I'll let you know how it goes with Erin's Mothership. Where's Souper Trouper?"

'She's having a small debate with our delivery boy over wonton ratios. The usual. Oh-oh hell no they're fighting now. They're fighting. Look, I've gotta save him before she gets him in a headlock- too late. I've gotta go, not sure the mayor will be cool with murder charges.'

Jillian smirked, shaking her head. Her foot hit the gas pedal, the hearse peeling away from the firehouse with a screech of rubber against concrete.

The drive wasn't a long one, and after half an hour of dancing in her seat to a CD that skipped every now and then (ectoplasm coating the dashboard proved to be a bastard, even months after that particularly messy bust) and resisting the temptation to turn on the siren and floor it, she had pulled up outside a tidy looking house.

A white picket fence bordered a short, well-maintained lawn. As Holtzmann's heavy footsteps crunched loudly down the picturesque gravel path, an uncharacteristic nervousness began to sink in. The end result would remain the same - Holtzmann would propose to the woman she was head over heels for - but she hoped nonetheless that Mrs Gilbert would approve of the proposal for the sake of her daughter.

She rang the doorbell.

She waited approximately five seconds before turning on her heel with the intent to drive far far away, happy to say 'Hey, I tried. A bit.' about the entire thing. Unfortunately, she heard the door open, and stopped mid-stride with a foot suspended in the air. Turns out both Erin and her mother had impeccable timing.

"If you're here to sell me more copper magnets then I'd just like to say that the last batch I bought aren't working too well."

Jillian laughed at the physics joke, turning to face the mother of her beloved. Realising the woman wasn't joking in the slightest, her laugh trailed off slowly and awkwardly, ending in a '...oh,' and a frown. Clearly Erin inherited her scientific mind from her father.

"Oh Jillian, it's you!" Erin's mother greeted, beckoning the blonde closer and enveloping her in an uncomfortable and slightly unwelcome hug. "Where's my little Erin?" She asked, despite being a good few inches shorter than Holtzmann, who wasn't exactly hitting in the big leagues height-wise.

"Y-you remember me?" Holtzmann asked, surprised. "And uh, she sends her love but she's... busy. So here I am, all business, no pleasure." She couldn't remember the last time she had to actively rein in her personality and try act 'normal'.

"Anyone who makes my daughter so happy is unforgettable. Please, come in."

Jillian followed her into a small sitting room, the television airing one of the ubiquitous daytime talk shows that infested networks like a virus. Perching uncomfortably on the very edge of a sofa and wiping her sweaty palms on her pants, she watched Mrs Gilbert sink into her own chair.

The woman's grey hair was cut into a sleek bob, her dress sense elegant and practical. Erin definitely got her facial structure from her mother, but her mother's eyes were completely different than those Holtzmann loved to gaze into, captivated, as though they were stars on a clear night.

She took the opportunity to look around.

Photos of Erin with both her parents were hung on the walls, from when she was a small beaming child with chubby cheeks, up to when she graduated, cap in hand and her father's arm around gown-adorned shoulders, smile not quite reaching her eyes.

Her heart leapt into her throat when she saw several framed pictures of herself and Erin on the mantelpiece: a selfie taken shortly after they'd saved the city; a picture of them on a beach in Southern California, when they'd had a rare holiday, their smiles beaming into the camera; a photo of Erin on Jillian's back, caught mid-laughter as her index fingers poked her girlfriend's cheeks where her dimples would be, the engineer crossing her eyes and pulling the goofiest face she could muster.

No wonder Erin's mom could recognise her - she'd seen her at her worst.

"She sends me these pictures," the older woman started, noticing where Holtzmann's gaze had gone. "And I feel so happy every time I look. She hasn't smiled like this since… well, since we abandoned her."

"I try my best, she deserves it," Jillian said sincerely, "Mrs Gilbert, I-" She stopped for a moment, saying the next words to herself more than the older woman.

"I'm going to ask Erin to marry me."

The engineer studied Mrs Gilbert's face carefully, looking for any sign that she disapproved. When a piercing neutral stare, remarkably like Erin's in its intensity - despite their eyes being completely different - met her own inquisitive look, Holtzmann felt her stomach twist into knots, expecting the worst, despite evidence to the contrary.

"Well it's about time."

She let out a breath she didn't know she'd been holding. "So you approve? Not gonna freak out and hit me with a decorative candle?"

"Of course approve, you kids these days take forever," Erin's mother said with a laugh. "My husband proposed to me after our third date and we never looked back."

Jillian didn't know what more to say, feeling awkward yet relieved.

The older of the two sighed, slowly getting to her feet and shuffling towards the television. Jillian watched curiously, wondering if the woman had finally had enough of the drivel playing on screen, raising an eyebrow and craning her neck for a better look when she saw her slide out a wooden box from a shelf.

"This here," she began, straightening her back with an audible pop and making her way over to Holtzmann, "is my mother's engagement ring. My fingers are like sausages, so I could never wear it. Of course you can say no, I'd understand, but I would love it if you proposed to my Erin with this. She has daintier fingers, but you'd know better than anyone about that-"

A thousand illicit images burned in Holtzmann's mind, and she cursed her filthy mind for conjuring such sins in the presence of her girlfriend's mother.

"-what with all that handholding you do."

Handholding, yes.

Realising Mrs Gilbert was practically holding the box under her nose, waiting for an answer, Holtzmann mentally shook her head of the thoughts, taking the box gently and popping it open.

The silver engagement band was simple, with three small diamonds embedded in the middle. She had already bought a ring, but looking at this one now, seeing how it had sentimental value that Erin would appreciate, she figured she would keep the other she purchased for herself.

"Are you sure?"

"I'd rather see this ring on my daughter's finger than trapped in a wooden box. My husband and I made some stupid mistakes-"

"Really really stupid," Jillian interrupted before she could hold her tongue.

The older woman had the grace to look slightly shamed before continuing. "And we hurt my daughter more than I could ever understand. She may have forgiven me, but I'll never forgive myself for putting her through those years of therapy. She's been happier with you than I've ever seen her, and I hope you'll save a seat for me at the wedding." She picked up an old photo of herself and Erin from the coffee table. Erin looked to be around seven, smiling at the camera with her front teeth missing.

"She once wrote this little list when she was young, and she said she wanted three things. One, to do science. Two, to have lots of friends. And three, to be married to her best friend. I'm glad I'll be around to see the third one happen, even if I'm not there in person. I was starting to worry I wouldn't be."

As much as the woman's apology was from the heart, Holtzmann's protectiveness of her girlfriend still gripped her, and she worked hard to plaster a smile across her face. "I don't want to take up any more of your time, I just thought I'd let you know. I'll see myself out."

When she was halfway down the gravel path, she felt a slight bit of regret being so curt with the woman. She turned to look back, facing Erin's mother, who had followed her to the doorway. "Oh, and if those copper magnet salesmen sell you any more faulty products, send 'em my way, kay?"

The chuckle she received in response signalled that perhaps Holtzmann could completely forgive Erin's mother eventually for what she had done, like Erin herself had.

Stage five: The trail.

The second-to-most pivotal stage of The Gilbert-Holtzmann Equation. The proposal itself hinged on the success of stage five, and as a result, Abby and Patty had done all they could to ensure it would run smoothly.

"I swear, I hate public transport. Why are all the drivers so angry?" Abby asked, relieved to finally see their home street.

"Trust me girl, if you'd done your service to public transport like I have, you'd know why," Patty responded, trying to repress the memories.

Entering the firehouse after their 'stakeout', the two eagerly awaited word from Holtzmann about her own stage of the plan. What they didn't expect however was to see Erin curled up quietly on the old beaten couch, knees tucked under her chin and what little mascara she wore left smeared over her cheeks.

"Erin!" Abby exclaimed, immediately running over to her friend and perching next to her. "What's wrong?"

"I told you. Totally told you. I told you to cancel the subscription to Lifetime-"

"I think I've upset Holtz," Erin admitted quietly. Abby and Patty shared a look, wondering how they were going to navigate this minefield. Abby sat back and pulled Erin into a warm embrace, just like she used to do when they were younger.

"Why would you ever think that?" She probed.

Erin sniffed, wiping her eyes on a tissue offered by Patty. "Today - this is too much information, I know - she completely rejected my advances. It… it sounds so stupid, I guess, but the last few days I've felt her distancing herself, like she's intentionally avoiding me. She'd get out of bed for hours and I'd see her sleeping on the couch in the morning. And earlier today, she went out dressed less like… Holtzmann. And she's been out for hours. Oh my god. What if she has found another woman?"

"Look, calm down," Patty soothed, hand placed reassuringly on Erin's thigh. "I'm sure Holtzy has her reasons. She's crazy about you, trust me, she never shuts up about you."

"I almost want verbal AdBlock for the name 'Erin Gilbert'," Abby said, relieved when she felt Erin smile against her neck. The physicist sighed, sitting up and rubbing her face.

"You know what? I'm just gonna do the adult thing and talk to her when she comes back."

"NO!" The other women shouted, making Erin jump out of her skin.

They chuckled nervously.

"I-I mean, just give it a few days, alright? Maybe see if it gets better?" Patty covered, eyes pleading with Erin.

Erin narrowed her eyes, suspicious. "Do you guys know something I don't? Is she hiding something from me?"

"She's just…" Both women looked to each other, willing the other to finish the sentence. Abby sighed. "It's one of those things that has to come from her. Please Erin, trust us. Wait for her."

Erin exhaled, trying to drop her natural desire for answers, and ignore the feeling of hurt that Abby and Patty knew something about the woman she loved before she did. They were like family - what could possibly be bothering Holtzmann that she couldn't be privy to?

"Now how about a movie?" Abby suggested, keen to move on. "Let's relax today, we've been kicking so much ghost ass lately. I'll go raid Holtzmann's mini fridge."

Leaving Erin in the capable hands of Patty, Abby dashed as fast as she could to Holtzmann's lab, slamming the door behind her and fumbling with her walkie-talkie.

"Holtzmann? Erin is real suspicious of everything right now."

'No code names. Must be serious.'

"She was super upset when we came back too, like, on a scale of one to ten, it was ruined mascara and self deprecating analysis."

'Well that's… that's problematic. Please say she's okay, I didn't expect to hurt her."

Abby grimaced at the waver in her voice, already seeing the upset puppy dog eyes Holtzmann was probably sporting. "She is right now, but we can't fend her off forever. You've got to activate stage five soon."

'Stage four was a success. Her mom even gave me granny's engagement ring, and I'm on my way back. Tomorrow morning, it starts. Let's do this, Yates. I'm trembling like an adolescent drenched in Axe deodorant an hour before prom.'

That night, Holtzmann made sure to cuddle Erin close, clinging on even when the heat of their shared bodies became almost unbearable. She lost count of how many kisses she had plastered the physicist with as they made love, hoping each one conveyed just how sorry she was for inadvertently upsetting her, and loving every single laugh that escaped her girlfriend at the abundance of affection.

Erin awoke alone.

Her heart sank momentarily, wishing that, after such a good night, her girlfriend had stuck around. 'She's probably busy working on something,' she mused, sitting up and stretching her aching body, looking over at the rumpled sheets where the blonde had been sleeping.

She yawned, making her way to the downstairs kitchen area to make coffee. The entire firehouse was eerily quiet - she couldn't spot Holtzmann or Abby anywhere, and she wondered why Patty, the earliest riser of them all, wasn't already sat on the couch slowly adjusting to the morning with a book.

"Morning Erin,"

"Oh God," Erin cursed, hand on her chest to still her beating heart. "Kevin? What're you doing here so early?" She pointed over her shoulder to the analog clock behind her with a thumb. "The short line is on the wrong half of the circle." Explaining the concept of time to the blond had been like pulling hen's teeth. "Where is everyone?"

"They told me to give you this," Kevin said, taking his mug off of a piece of card he was using as a coaster. His tea had left a ring on the surface. He handed it over, a pleased smile on his face.

Erin plucked it from his fingers, eyeing it warily. Was that a clear earpiece in his ear, too? She had a feeling everyone's weird behaviour would soon be explained. "Thanks Kevin."

"No problem. Once you're done with that, they said to meet me outside." And just like that, he let himself out with no further explanation, just as he always did. Erin honestly wondered how he had functioned in life up until this point.

Once the door had closed, she read the card.

To where it all began.

She frowned, flipping over the card, disappointed to find it blank.

'Riddles,' Erin thought, rolling her eyes. She hadn't had her coffee yet, and it was too early. Still, she had a trail to follow, and her mind raced for explanations for its conception. It wasn't her birthday, so it couldn't be a lead up to some extravagant gift. Nor was it near enough Christmas to entertain gift-giving. She sighed, heading upstairs once more to get dressed and board the crazy train.

When she got outside, she was greeted by the sight of Kevin on his motorcycle, the engine running and his helmet already secured in place.

"Hey Erin, wear this. Safety first." He tossed her a helmet, which she looked at, confused.

"Oh, guess we're going for a ride now then. Great. Erin Gilbert, MotoGP champion of twenty-sixteen," she announced sarcastically. She wasn't the biggest fan of motorcycles, but pulled on the helmet regardless, gingerly mounting the vehicle behind Kevin. He kicked it into gear and they were soon racing down the streets of New York.

Once upon a time, being in this position, arms wrapped around Kevin Beckman's waist and smelling his fifteen dollar cologne, would have been Erin's dream during her short-lived fancy of the man. But now all she wanted was to go back to bed with Jillian Holtzmann, rest her head on her chest and listen to her heartbeat, lulled to sleep by her comforting scent - something earthy and fresh, like pine, mixed with a hint of oil.

They finally came to a stop outside of the college she had first confronted Abby about their book. 'Of course, where it all began.'

"First stop, off you go," Kevin ushered happily, holding onto her helmet for her once she had dismounted.

"Why am I here?" She asked him, knowing it was futile. He shrugged, dashing that last bit of optimism. She patted him on the shoulder. "Thanks for the help, Kevin. Don't stress yourself out."

She entered the building, trying to remember exactly where Abby's office - if you could call it that - had been on that fateful day when the Ghostbusters had been born. Staff members looked at her quizzically, some nodding in recognition, but never stopped her progress.

She came across a group of people stood outside the door of what she believed to be her destination. Frowning as she approached, she could just make out the words of what seemed to be a tour guide.

"...and here we have the old office of two of the Ghostbusters, Dr. Abigail Yates and Dr. Jillian Holtzmann. Before they rose to prominence in the battle of New York, their extensive research into the paranormal was actually funded by this very institute, and we heavily supported their work and subsequent discovery of ghosts." A balding man said, dressed in a green plaid shirt and mud-coloured pants, his audience captivated.

'More like you had no idea they still used that lab,' Erin thought, miffed that the college was trying to attach itself like a leech to the Ghostbusters' achievements. 'But yes, let's say you funded everything, and didn't tell them to get the hell out, and didn't spell science with a Y, and didn't know that was wrong.'

"Oh my, ladies and gentlemen," the guide said, suddenly looking at a spot past Erin's shoulder. The gazes of his audience followed. Erin looked over her shoulder for the source of the interruption, confused.

"It's Erin Gilbert!" Someone whispered excitedly, and Erin's face dropped. She still wasn't used to this quasi-celebrity thing.

"Ms. Gilbert, how lovely for you to join us," the man said, probably thinking his leery smile was charming.

"Doctor," Erin corrected coolly, half-heartedly waving to the admirers. "Doctor Erin Gilbert."

The whispers intensified, and she felt as though she were an insect in some sort of exhibit, being looked at through a magnifying glass.

"What brings you here, Dr. Gilbert? Come to look around our fine institute?"

"You could say that." Erin walked through the crowd, which parted obediently for her, towards the door, noting that it had name plaques for both her friends, as if it had been a renowned office, and not simply where two kooky friends did their best to unveil the paranormal. She put her hand on the handle of the door, hoping they'd catch a hint. "Now if you'll excuse me… I've got some ghost stuff to get."

"I'm sure the Ghostbusters are very busy, so let's allow Dr. Gilbert to get to work."

Despite the disappointed grumbles, the crowd moved on with the tour, leaving Erin blissfully alone. She sighed in relief, pushing down the handle, only to slam her shoulder into the wood when it didn't yield.

'Locked?'

She could hear footsteps patter closer and the door latch unlock, opening to reveal Abby Yates.

"Oh Erin thank god, I've been waiting for those Ghost-nutters to leave for ages, I thought they'd never go. That's why I locked the door." She pulled Erin into the old lab, which was empty aside from Abby herself, and a CD player next to her feet.

"Um, what is this all about?" Erin asked, looking at her friend expectantly.

Abby's face dropped. "Oh shit, they put me off. Damn it! Okay okay, just imagine you've just walked in, and I'm stood right here, in the middle of the room, and I've just said 'come here often?', just like Holtz did all those years ago. None of that just happened."

"...okay," Erin said, confused. She watched Abby pull another card from her pocket, reading its contents, her lips moving as she rehearsed. She bent to press play on the CD player. DeBarge's 'Rhythm Of The Night' filled the room, and Erin couldn't suppress an affectionate laugh, her hands covering her face as a blush filled her cheeks, eyes peeking through opened fingers.

"When you first came into this room," Abby started, "I felt my entire world light up like a nuclear reactor had gone off in my heart - in a really good way." Her friend stopped for a moment. "This is from Holtz by the way, I'm sure you guessed but I'm not taking any chances; I'm not declaring my undying love for you."

"Thanks for clarifying," Erin said sarcastically, smiling fondly. "I was worried for a second there, obviously."

Abby cleared her throat. "Rocking the tweed and looking angry as hell, I knew you were my kind of woman."

It took all of Erin's composure not to cry in front of her friend; she didn't even know why she felt suddenly sentimental.

"That was so cute," she finally managed, "now where is she?"

"Not so fast," Abby answered, handing Erin the card she'd read from. "You'll have to find her."

Flipping the card over, another clue stared back at her.

To where you started again.

"What?" She asked aloud, brow scrunched in confusion. She looked to Abby. "What does this mean?"

"Can't do it for ya, Holtzmann's orders," Abby responded unhelpfully. "Now I've gotta jet, you remember that the door locks behind you, right?"

"Of course," Erin said with a smile, watching her leave. She was suddenly transported back to that fateful day three years ago, and Abby's words stuck with her.

'Wait… the door locks behind you… I went with Abby and Holtz to the Aldridge Mansion! That must be it!' She concluded, rather proud with herself. 'And I… started again. Saw a real ghost, left Columbia behind for good, and helped form the Conductors of the Metaphysical-' She cut herself off mid-thought. 'Oh forget it. Ghostbusters. We formed the Ghostbusters.'

Quickly scanning the room for anymore hints or clues, she left for the entrance, managing to avoid the group of fans from earlier. Kevin sat patiently on his bike, and once she donned the helmet they were soon screeching off again. She wanted to ask him again what was going on, but knew that with the helmet on and the wind all too eager to carry away her words, and Kevin being Kevin, it would be a fruitless task.

Despite Kevin's careful riding, as if she were precious cargo, it didn't take too long to arrive at their destination.

The mansion looked just as it did all those years ago, and the same tour guide who had been terrorised by the first ghost they had seen together as a unit was there to lead her in.

"Ms. Erin," he said, head bowed with a smile. She didn't bother correcting him, simply pleased that she had followed the clue correctly. "Welcome to the Aldridge mansion. Hopefully it's less terrifying the second time around."

She took the time to look around the place, remembering the feel of ectoplasm coating her like a second skin but elated to know that she and Abby were right, ghosts were real after all. Holtzmann had been content to eat her salty parabolas, entertained yet unphased by the malevolent entity.

The guide cleared his throat, card in hand. "Your aesthetically pleasing yet practically useless tiny bow ties may not hold your collar together, but I think the one you wore this day tied my heart to yours."

"Oh god," Erin muttered. "She's my girlfriend. She wrote that. For me. Can you believe it? What a dork, right?"

The guide looked at her, shuffling uncomfortably and not knowing what to say, so instead he just handed her the card.

To where we met Kevin

Erin raised an eyebrow. 'Above the Chinese restaurant?'

Letting the guide lead her out, she and Kevin were soon on their way again, leaving her to wonder exactly why Holtzmann had gone to all this trouble. Perhaps an apology for her distant behaviour? Not that she needed to apologise - Erin was perfectly content to let her have her space if required. But the engineer always did go adorably over the top with apologies.

She recalled the time when Holtzmann had accidentally clipped Erin with a proton blast during a bust, and though it singed her coveralls slightly, no actual damage was done. Holtzmann nevertheless spent the next three days doting on her, following her around with a heartbreakingly sad look on her face saying 'sorry' continuously until Erin kissed her senseless to shut her up.

'That was a good week,' she mused, realising she probably shouldn't be daydreaming about kissing Jillian Holtzmann whilst whizzing through the city streets, head absentmindedly pressed against Kevin's broad back, lest she accidentally let go and fly off the bike like a helium balloon.

Patty was waiting outside the restaurant when they pulled up outside, and Erin was almost immediately pulled into a warm embrace as soon as she pulled off her helmet by the taller woman.

"What's all this about?" She asked, almost pleading for answers at this point as they broke apart.

"Oh nuh-uh, you're not at the end yet, and I'm not into spoilers," Patty responded, "now we can't actually go into our old place, since someone lives there and all. But this is good enough."

She cleared her throat, producing a familiar pink card.

"This is where I watched you lust over our resident beefcake like you hadn't eaten in weeks," Patty started, giggling through her words as she watched Erin hold an embarrassed palm to her face. "And the entire time, I wished I could have been a continental buffet before your hungry eyes- Oh hell no, did she change these at the last minute? I can't believe I said those words with this mouth."

Erin snorted at her friend's reaction, so delightfully Patty in response to something so delightfully Holtzmann. She wouldn't be surprised if Holtzmann had changed it on purpose just to mess with her.

"Now I've been scarred for life you can go ahead and get outta here, even I want you to find Holtzy at this point. I gotta go swill some holy water."

She accepted yet another pink card, flipping it over.

Her next clue took her to the Mercado Hotel where they had staged the last showdown against Rowan; she was offered a glass of champagne, treated to a large group of people dancing with and around her, and most importantly, her final clue.

Go back home.

Stage six: Put a ring on it.

It was dark out by the time she'd finished trekking the city following clues. The firehouse was quiet when she entered, and she could just hear Kevin's motorcycle rumble into the distance outside.

"Hello?" She called, almost expecting her friends to pop out from behind the counter to surprise her.

They didn't.

Peeking into each nook and cranny, calling all of their names, she was met with the same silence. She briefly wondered if she was thinking of the right 'home' the clue referred to, unable to conjure up an image of any other place.

The last place to check was the roof. She pulled on a jacket as she made her way up the stairs, praying that she would finally see her girlfriend after a long day without her. Erin was just about ready to sink into her embrace.

When she emerged, the first thing she could see was Holtzmann's back, the engineer leaning on the railing as she overlooked the streets below. Just seeing Jillian stood there in all her eccentric, beautiful glory was enough to flood Erin with relief.

"Holtz?" She called out.

Holtzmann didn't turn around. "It took you exactly ten hours, five minutes and twenty-one seconds to reach here. Did somebody enjoy the champagne and flash mob a little too much?"

"There's a thing called speed limits," Erin retorted, wrapping her arms around herself to ward off the chill, smiling fondly nonetheless. Holtzmann really had gone all out for whatever this was. "But yes, it was highly entertaining, and I would recommend it to my women's book club."

Holtzmann chuckled, a rare sound (she preferred to express mirth through her face) and music to Erin's ears.

"Speed limits are guidelines."

"Guidelines to you, maybe. The rest of us call those guidelines laws."

Holtzmann finally turned to face Erin, holding her hand out to beckon the physicist closer. Her cat-like eyes shone in the light from the roof lamps she'd recently installed, shadows enhancing the strong planes of her face and curve of her dimples. Erin wasn't sure she'd seen anything quite as beautiful.

"What's all this for?" She asked quietly, afraid that if she raised her voice she would ruin the magical feeling. Her soft hand slipped comfortably into Holtzmann's callused counterpart, the engineer's other hand finding its way to Erin's waist beneath the unzipped jacket. They began to sway to a beat only they seemed to hear, Holtzmann's shorter height giving her the perfect access to Erin's neck, where she would sporadically place gentle kisses.

It was rare for Holtzmann to slow dance. 'Why dance slow when it's so much more fun to throw your limbs around like you're hooked on a ceiling fan?' Was her excuse, and being here with Holtzmann now, Erin could say she was glad that this wasn't a regular occurrence. It felt special.

"What if I told you that all of this was for no reason?" Jillian finally responded, her grin plastered against Erin's neck already giving her away.

Erin laughed. "I'd say you were a liar."

"I'd say you might be right."

They swayed for a few more minutes, before Holtzmann finally stepped out of the embrace.

"You're probably wondering why I've brought you here,"

"Jeez, I did wonder if my million questions to that effect gave it away,"

Holtzmann grinned. "The places you've been are where my favourite memories happened. Meeting you, our first ghost, all that good stuff. Makes me all gooey inside to think about. I'd have done our first date and everything but I ran out of people."

"It's been perfect," Erin reassures. "Like, amazingly perfect, I've had such a good day. But what's your favourite memory here? You never said." She watched Holtzmann's smile tighten slightly. The blonde cleared her throat, breath slightly ragged.

"I could say my favourite memory of this rooftop was standing here as the Ghostbusters after saving the city - the world - from a megalomaniac with a superiority complex. But that would be a lie."

Without a further word, she produced a ring from her inside pocket and dropped to one knee, stumbling slightly with an 'oops' before correcting her balance.

"J-Jillian," Erin whispered, heart lurching and trying to process what was happening. She never used to understand why people would cover their mouths and immediately begin crying when proposed to - until now.

"Three years ago, I made a speech to my new friends about how thankful I was, because I'd never had what you all gave me. And now this speech is just for you, Erin."

She closed her eyes, recalling the words she had carefully crafted and memorised, her knee starting to hurt on the concrete floor but unable to care enough to shift the pressure.

"I'd never known true love until I fell for you. I love your tiny bow ties, I love the way you get frustrated at the smallest of things, I love the way you dance with me like nobody's watching, I love everything. I never thought I would be able to say those three words like this. That's why you deserve the truth. Erin, my favourite memory of this rooftop is proposing to you - will you marry me and be my hot science wife?"

Not even a second passed.

"Yes, yes, god, yes! I'll be your hot science wife! I'll be your hot regular wife! Any wife!" Erin answered, crying freely now as Holtzmann stood up and hugged her close, picking her up and spinning them around. She did her best to savour the moment she never thought she'd see. They kissed hard at first, caught up in the moment, before slowing to gentler kisses. Holtzmann's thumb wiped away the wetness on Erin's cheek.

Forcing herself to pull away, the engineer lightly grabbed Erin's left hand. "Are you ready to be dearly betrothed to a sexy, smart, amazing, hilarious-"

"Don't keep me waiting anymore, please," Erin begged, trembling, fingers already outstretched. "It doesn't feel like this is real."

Holtzmann slipped the ring on, sealing the contract they'd just made.

"Well I'll be darned," Holtzmann started, looking at the ring. "It's a perfect fit. My Cinderella wears a lot of tweed and kicks ass, who'd have thought?"

"I really want to talk about how this is my grandmother's ring, and how it completes the most amazing day of my life so far with you, and how I wonder where you got it from, but I want to kiss you more," Erin admitted, closing the gap between them and silencing any of Holtzmann's witty responses.

Just as the kiss was becoming more heated and passionate, the walkie-talkie hooked to the back of Holtzmann's pants crackled to life, breaking them apart.

"Holtz? You've been quiet for a while, we're waiting for news here," Abby's voice said.

"Did she say yes? I don't care about anything else, Holtzy. Did. She. Say. Yes?!" Patty demanded from the background.

Erin smirked, finally understanding a more complete picture of her friends' strange behaviour. Before Holtzmann could grab for the device, the physicist reached behind her and swiped it.

"She said yes," she spoke, relishing in the surprised pause before laughing at Patty's eventual 'I knew it! I knew you'd say yes, Gilbert!'.

Erin turned her gaze back to an especially proud-looking Jillian Holtzmann, just standing there with her unkempt hair and irresistible grin and adorable dimples. She lifted the device back to her mouth. "She also says stay clear of the firehouse tonight. Your ears will thank you."

Dropping the walkie-talkie to the floor, Abby and Patty's complaints of 'TMI: too much ick, Erin' and 'you nasties better keep it isolated to your room!' completely ignored, she grabbed Holtzmann by the front of her shirt, pulling her into a searing kiss before leading her back downstairs from the rooftop, beginning a new, exciting chapter for both of them.

Later, when holding a sleepy Erin in her arms, Jillian would muse that 'The Gilbert-Holtzmann Equation' had been solved for x, where x was a frickin marriage proposal and y was a big fat yes from the love of her life.

This became so much longer than I anticipated. It was meant to be around 3k words, maximum. Clearly not the case now.

When I started, the Holtzbert engagement fics hadn't started yet on AO3/Tumblr etc, and now I'm just kind of late to the party. I hope you enjoyed this, I know I loved writing it, even though there are parts I'm not super happy with, and I only watched the film once (criminal, I know), so characters are probably OOC at some points. Just wanted to get it done eventually.

Also written on an iPad with a Bluetooth keyboard since my laptop broke - please disregard any spelling/grammar errors, they aren't intentional, this hellbound device just wants to write for me.

I'd upload this onto AO3 but I don't have one! So that's why it's only on my Tumblr sideblog linked to the cesspit that is .

Until next time!