A/N - So here I am back at last with a new multichapter. It's definitely a smaller project than DttW, but I hope you'll still enjoy reading it. This idea is not particularly original and I know it's been already done, many times over probably, though I don't recall ever reading anything strictly concentrating on this. Either way, there is a reason why I picked this topic (apart from obviously wanting very, very badly to write it), which will be revealed later in the story. But enough of the boring stuff for now, read and let me know what you think.

Also, Nik47, you are an angel in the form of a skilled, wonderful, patient beta!

Set somewhere starting S5 probably, expecting Castle's and Beckett's relationship is still a secret.

RULES OF ENGAGEMENT

Chapter 1

"You have to get a grip on yourself, Castle," she growls, her hands squeezing the steering wheel.

"It's not like anyone has noticed, Kate. Relax," he drawls smugly, that careless casualness of his tone driving her over the roof.

She's angry with him. Well, not angry really, but definitely irritated. He's been reckless at the precinct lately, stealing touches and caresses whenever he thinks no one's looking, always smiling just a little too brightly, grinning and teasing her a tad too saucily.

As of yet, they've managed to keep their relationship a secret, but that's due more to luck than anything else. And at the rate Castle's going with his open, shameless flirtations, Kate doubts they'll last another week.

"Why can't you just keep your hands to yourself?" she bursts out, her eyes leaving the morning traffic briefly in order to scowl at him.

"Hey!" he protests loudly, his eyebrows shooting skyward in disbelief at her accusation. "You've been the initiating tease in at least half of the incidents under review!"

She snorts indignantly. "Yeah, right."

"No, I mean it, Beckett!" he defends. "What about those vicious little smirks you're always throwing my way, or that sultry wink you sent me the other day? That was not cool!"

Okay, so it's possible she might share in a small amount of the blame here. But still.

"Yeah, well, we'll just have to learn to control ourselves better when we're around each other at work," she states, leaning into a sharp right turn, her cruiser smoothly following Esposito's around the corner. "We've gotta be more careful, Castle. Lanie's been sniffing around for weeks, demanding to know who the mystery guy I'm supposed to be having sex with is," she complains, glancing over at him again. She spots that annoyingly dreamy and oh-so-satisfied grin blossoming across his face at her words, and she aims a slap at his chest.

"Stop it!" she admonishes, glaring as his grin merely widens. "I'm serious, Castle. If Gates gets wind of this, you're out of the precinct for sure!" That finally seems to sober him up. Good. He needs to get real about this, because it's important, and she's not willing to lose him. He's her partner, in every sense of the word, and she needs him.

They come to an abrupt halt, parking close behind Esposito and Ryan. Throwing a last warning glance in Castle's direction, she retrieves their safety vests from the back seat, shoving his a little too roughly into his chest.

"Now put this on, and act normal!"

He pouts at her, snatching the vest from her fingers before stepping out of the car and sulking his way over to the boys. He studiously ignores her as she joins them, throwing her own vest over her head and pulling the Velcro straps closed.

"I still can't understand why we have to put these on," he grumbles, refusing to look at her, directing his question towards Ryan instead. "We're just canvassing. Knocking on doors, asking questions. Where's the danger in that?"

She knows he's just trying to provoke her, knows he's still irked about her comments in the car and looking for a bit of payback, but he's just exceeded her patience limit, and she's done. She cuts off Ryan with a gesture before he can field the question and rounds on Castle, poking her finger into his unprotected chest.

"In case you haven't noticed, Castle, we're not in Manhattan anymore. Not that that matters, because it's protocol. You can get yourself shot on your own time, but on my clock, you follow my rules. Now shut up and put your damn vest on, or you're staying in the car." She shoves him one more time for good measure before marching off.

Ryan and Esposito exchange a smirk before turning to Castle with raised eyebrows, blatantly slipping on their own Kevlar as they wait for him to make a move. He narrows his eyes at Kate's back, frowning, a tinge of challenge still running through his veins, defiance clearly evident in his glare.

She's obviously upset with him, but she's also being completely unfair. He's not the one who accused her of jeopardizing their relationship after all. And he definitely didn't deserve the verbal or physical lashing she just gave him, in front of the boys no less! Still, he doesn't dare to object, not when she's this angry and obviously serious as a heart attack.

He finally gives up, petulantly pulling the vest over his head, all the while ignoring Ryan and Esposito's badly masked snorts at his apparent cowardice.

"There, happy?" he mumbles childishly to no one in particular as the boys set off towards Kate. He jerks the Velcro into place and follows them. "Can I go play with the other kids now, Mom?" His snarky comment is just loud enough for all three of the waiting detectives to hear, and Esposito can't help a snigger, seeming to drive Kate's irritation levels even higher.

"Sure you can, Ricky," she throws over her shoulder in a sugary voice. "Just play nice and don't steal Espo's toys. He's particularly attached to his tea set."

She jerks open the door and disappears inside, ignoring Esposito's indignant 'hey!' and Ryan's amused snort.

"Shut up!" the Latino detective growls at his partner before throwing a dark look in Castle's direction. "Let's go," he commands grumpily.

They've got testimony from their vic's cab driver claiming he picked her up from this apartment building in Brooklyn Heights before dropping her at a street corner in Lower East Side where she was later killed, but they've yet to find out why she was here in the first place, or who she was meeting with. All they know is that the cabbie described her as being 'extremely angry and upset' when she hopped in his car. So now they get to go door-to-door, flashing a photo of their vic and asking residents for any information they might have. The hope is that they'll find someone who might have seen or heard something that night, something that brings them closer to cracking this so far fruitless case wide open.

They start at the top of the building and work their way down floor by floor, apartment by apartment. Half of the doors remain firmly closed though, and those which do swing open mostly reveal the scowling demeanors of not-too-cooperative residents, all of whom are unwilling to talk once they spot the police badges on display.

So far they've found nothing, and Kate is starting to reconsider the intelligence of this move – if their killer does actually live here, they might well be giving themselves away. Hell, they probably already have. Still, they're nearly halfway through, and the element of surprise is pretty much blown anyway, so they might as well finish the whole building.

Castle is now three doors to her left, and Ryan and Espo are interviewing the residents of the apartments in between. Surprisingly enough, this particular corridor has been more cooperative than the rest, its residents actually striking up conversations with the inquiring detectives. Unfortunately, many of the interviewees have simply taken advantage of this opportunity to complain about their neighbors, or the high crime rate in their community. No one seems to know a thing about the victim herself, and the detectives are still completely empty-handed.

Beckett sighs as another door slams in her face then turns to watch her colleagues' progress. She can hear Castle banging on the next door, his voice booming over the boys', and she doesn't know whether to roll her eyes or go over and smack him upside that pompous head of his at hearing his words.

"NYPD, open up!"

Okay, so he's obviously still sulking at her earlier reprimands in the car, along with those in front of Ryan and Espo, because this is just another petty display of rebellious defiance. He knows only too well how much she dislikes it when he announces himself as NYPD. He is not a cop. Despite his great achievements with the force, he's just not a cop, and Kate hates the fact that she has to sometimes remind him of that. He's a civilian consultant, without any real authority or power, without a gun to protect himself in times of need, and he should never forget that, for his own protection as well as for the safety of them all.

She chooses to ignore it for now, making a mental note to tell him off later, concentrating instead on her next door, which doesn't open even after several loud raps from her knuckles. She gives up and heads farther down the corridor, stepping around the boys in order to try her luck with the last apartment at the end of the hall. She's still only halfway to Castle, just passing an exasperated Ryan, who's being chatted up by an old lady cradling a cat in her arms, when the door the writer's been banging on whips open, throwing him slightly off-balance. She smirks automatically at the sight of her partner – his fist still hanging in mid-air, eyes wide and mouth open in surprise. And there's no time for her to even analyze that flicker of fright which flashes in his eyes before three gunshots blast through the air, bouncing and echoing off the walls of the narrow corridor, three flares of painful light slicing through the dimness, momentarily blinding Kate's view.

She instinctively drops to the ground, protecting herself as best she can in the coverless space, crouching into a minimal exposure position and curling into a tight ball of limbs and flesh as she launches herself to the side, the deafening sounds continuing to reverberate through her skull. Her shoulders hit the cheap wallpaper of the corridor at the same time she draws her gun, her eyes scanning the narrow space. Her gaze rises just in time to see the whole scene unfold before her in terrifying clarity: Castle's back slamming hard against the wall opposite the apartment's open door, the same door the shots came from. There is the shortest of moments when everything stills – silent and motionless, and Kate swears she can hear both their heartbeats – before his body crumples heavily to the floor, his legs giving out and his long, muscular frame sliding limply down to the ground, limbs sprawled out in all directions, eerily still and unmoving.

Kate's heart stops, just stops, with uncontrolled dread, and a single choked cry of pain catches in her throat, the sound never making it past her paralyzed lips.

A/N: Yes, there is more where this came from.