The Routine:

She hadn't meant for it to become a routine. When she'd kissed Sheldon on the fire escape, it had been a way of saying Thanks for saving my ass! I'm glad I'm not zombie food! Kissing had always been no big deal to was a way to say thanks, to reward a job well done. It was a hello, a goodbye, an invitation for something more. It had never really occurred to her that others took it as more. Sure, Leonard usually got that starry-eyed look whenever she kissed him, and then mooned after her for days in hopes of receiving another. Howard was, of course, taking every opportunity to present himself available for physical intimate contact of any kind. On the rare occasion Penny actually kissed or hugged Raj, he would usually turn an odd shade of purple and then scurry off somewhere else to either rejoice or panic - probably a little bit of both, if she had to guess.
But Sheldon...

For Sheldon, kissing became a retreat It was a physical act that allowed him to momentarily let go of his worries, his unspoken fears -his brain - and simply feel. Being who he was, this was a pleasure not often experienced, akin only, perhaps, to the Friday night Battlestar Galactica episodes he allowed himself to get sucked into. In the face of such a horrifying and cataclysmic world-altering event, he decided to allow himself a few more pleasures outside his normal realm of entertainment, especially since he wouldn't get to watch a re-run of BSG any time soon. Or ever.

Kissing Penny had been the next logical step, once he decided he would use that form of stimulus to escape. She was physically pleasing to look at, after all, and also the only woman in the entire city he know wasn't infected. Her proximity was convenient - he wouldn't have to dodge roll his way around the city just to see her. Then there was that little matter of the way the sun seemed to explode behind his eyelids the first time they kissed, but that was only a secondary requirement. The only complication he could foresee was the other men getting jealous. Despite his certainty on the matter, the others didn't see him as the alpha male, so he had no claim to her in that respect. Not to mention if he even attempted to use the words "alpha male" and "subservient female" around her, she'd most likely remove his testicles with that gun she knew how to wield so well.

He didn't exactly like the idea of the others using her in the same way, however, nor did he suspect she would relish in the idea, if her constant threats towards Howard were any indication. Stealth would be of the upmost importance. On the one hand, it would be difficult, since they all now occupied close quarters and privacy was very limiting. On the other, there was a whole city full of emptiness he could take advantage of.
Sheldon attempted to rationalize his thinking over the course of about three days, trying to come up with a reasonable way to explain to Penny why he intended to use her as his new form of escapism. When no sound theory presented itself, he instead did a most un-Sheldon-like thing, and acted.

No thinking.
No rationalizing.

Raj and Howard were scouting the area for more supplies, and Leonard spent the night with the Dorsin brothers, a pair of survivalist twins from three blocks down who, like them, decided to wait out the invasion in the comfort of their own heavily foritfied apartment. "Forging allies", as Leonard liked to put it. This put Penny and Sheldon alone in the apartment for the first time in a very long time. He found her curled up on the couch, staring at the ceiling as if it were stars. Her head rested comfortably on her folded arms, and her thin bare legs stretched down the length of the couch. At that moment, he didn't matter so much that she was laying in his spot. When she caught sight of his tall form looming over here, she smiled up at him.

"Hey Sheldon," she stretched like a cat, long limbs bending against sunlight. "I think I dozed off there for a second."
Don't think.
He sat down on the small sliver of couch her body didn't touch, and gripped the back of the couch and the armrest as if they were his link to sanity.
Don't think.
His agitation had her eyes shifting in concern, and she started to sit up.
Don't think.
He leapt.

Before she could speak or he could change his mind, he closed the space between them and crushed his mouth down on hers. He was too swamped with nerves at first to enjoy it. A second passed, then two as he waited for her to smack him or (hopefully!) kiss him back. When nothing happened, he slanted his mouth, kissed her again, softer this time. Just like Apollo and Starbuck, he told himself. Just like them. Then she broke, her lips moved back, and he felt himself sucked into heaven again.

That peaceful, warm place that erupted out when their lips touched swallowed him up. Only her mouth against his remained, only her fingers that curled against the back of his neck. His mind didn't try to rationalize or quantify. His brain no longer listed off the constant threats that plagued him, or the things he should be worrying about. He simply felt.

He felt good.

When attempting to explain the situation to herself later, Penny would chalk it up to being desperate. It had been a long time since she'd had any itches scratched, and with the alternating boredom and adrenaline that was constantly flooding her system in the last three weeks, she was very very itchy.

"You are simply replacing the stimulus I formerly used to enjoy myself," he fumbled over an explanation, with his brain still fogged from their kiss. "I no longer have the power of the TARDIS, vipers, and wormhole technology at my fingertips, so I need you." Looking away, he added, "For some reason, kissing you makes me this whole zombie situation seem less... overwhelming."

She should have been offended, since she was basically being used as a toy for his pleasure. Tumbling awkwardly from Sheldon, the words didn't seem quite so offensive, plus the last bit made her heart do a nice little flop in her chest. She chose not to dwell over it, but instead grip his face between her hands and kiss him harder than she'd ever kissed anyone. If he wanted escapism, she'd show him escapism.

Those minutes (or, okay, maybe more like an hour) on the couch rapidly turned into habit. Never on the couch again, not unless they were sure the boys were gone for the day, but they would find other ways to sneak a kiss or two. Or lots more.

The Apartment:

He came home one night, covered in blood, bits of someone else's spleen stuck fast to his sleeve. Raj was just as gored up, and had defined teeth marks all up and down his paint ball shoulder pads. Leonard stood at the ready with towels (to wipe away the gore) and a gun (in case anyone was bitten.) Howard got right to work picking off various extra body pieces that had gotten stuck to his friends, and dropped them in a bowl. They would be used later as bait. The minute Sheldon stepped through the door and focused his darkened eyes on her, she knew what her job was going to be.

She too had experienced the terror of a swarm of zombies as they stumbled down an alley towards you. She'd seen the light leave someone's eyes as zombies reached their jugular and pulled, hard. She'd lost too much sleep over it, and knew Sheldon didn't want to lose any more. As Leonard and Howard pecked at him like mother hens, he kept his eyes only on Penny, and she didn't dare look away.

"I'm going to see if there's any more water in the traps on my fire escape." she murmured. Leonard's eyes perked up.
"Do you need any help?" he asked, desperate for some alone time with the woman he loved. She just shook her head, grabbed her gun, and set about undoing the locks.

She slept at the boys' apartment because it was safer, and because no matter how much she'd scrubbed, she couldn't get the blood stains out of the floor, and that totally freaked her out. But they kept her place at the ready, storing extra supplies there and the like. It wasn't a place she liked to be alone, thanks to what happened last time, but occasionally it was nice to have a safe(ish) place away from everyone else to just think. She had made it her job to keep everything in order over there, since it was still technically her apartment. It had also become a nice place for her and Sheldon to escape to when things got... tough. And today, things were definitely tough.

She shut the door but didn't lock it, then sat down on her couch and waited. With every creak of the apartment, she jumped, remembering the last time she was in her apartment without locking the door. The boys had long since set up early warning systems that would trip should any zombies enter the building, so she knew she was okay. Still, the waiting was tense.
Seconds went by. A minute. Two minutes. What was taking him so long? Could she have mis-read the look in his eyes? Maybe he needed to be alone...

Then his knock came at the door.

Knock knock knock.
"Penny."
Knock knock knock.
"Penny."
Knock knock knock.
"Penny."
In a whispered voice.

"It's open," she hissed back. The door swung open, and Sheldon stepped inside, closing the door quietly behind him. He focused all his energy on the lock for a moment, sliding it into place, then turned to her. Most of the blood had been wiped away, and he'd changed his shirt so it wasn't covered in the stuff, but the hurt was still just as strong in his eyes.

"Oh sweetie," she murmured, and held out her arms. In two strides, he crossed to her, and crushed himself against her. She'd expected her face to be eaten off, perhaps even sex, but what she got shook her to the core. Sheldon sank to his knees in front of her, shaking, and pressed his face into her stomach. It wasn't until she felt the moisture seep through her tank top that she realized he was crying.

Sheldon? Cry? She hardly thought it possible. Explode, maybe, but cry? As she too sank to her knees and wrapped her arms around his trembling form, she felt her heart begin to crack. His face was distorted in the pain of all the lives he'd watched disappear. She leaned them both against the couch, feet tucked under her as she rocked him back and forth. The angle was awkward, he was taller than she, but he bent and clutched and sobbed so hard it didn't seem to matter.

She began to mumble nothings, more to calm herself than him, since he was too far gone to hear her, and pressed wet kisses onto the top of his spiky head. She felt herself falling apart at the mere sight of him. If a robot like Sheldon was coming undone, surely she had no chance at all. Tears for his pain, and the pain that now lingered like a fog over the city, spilled silently down her cheeks. There were no sobs, no wracking cries of her own, just the tears that mingled with his.
He reached up a hand and grabbed the front of her tank top, dragging her mouth down toward him.

"I don't want to feel any more Penny," his voice was ragged from crying. "Help me to not feel."

Ten minutes, two orgasms, and one very destroyed bra later, they lay sprawled out on her rug, wedged between the sofa and the coffee table, both completely unable to move. Zombies could have kicked in the front door and neither of them would have bothered to get up. Sheldon stared up at the ceiling, simultaneously confused and astounded by the events that had just occurred. He'd had sex before - one does not grow up in a small town without having a woman's lovely lady bits shoved at one as soon as puberty set in. He'd just never had Penny sex before. Penny sex made thinking absolutely impossible, even if he'd wanted to.

Penny sex took the sun that had previously exploded behind his eyelids, gathered up the pieces, set it ablaze, and burned it hot and bright all over his body before making it explode all over again. Whatever heaven he was disputed wasn't there, he now firmly found in her body.
Kissing would not be enough anymore, he knew. Oh no.

"That was much more effective in quieting my constant stream of thoughts." he stated, quite matter-of-factly. Penny, who until this point had smooshed herself as close to the couch as possible, ass pressed against the rough fabric, in case Sheldon needed his space, leaned her head against his shoulder, and wrapped an arm around his thin waist. His long fingers abscently traced the smooth skin of her arm as he took a moment to study the intense relaxation his body was now dipping into.

"Is there any chance we could repeat this process?" he asked quiety.
She leaned up, and nipped his ear, lips forming into a smile as his breath hitched.
"Honey, we can repeat this process as many times as you'd like."

The Dawn:

Blood seemed to drip from the sky, blazing red and beautiful as the sun rose over the city. It had been exactly two months and twelve days since the first zombie attack had occured, and finally help had arrived. Five people stood in their apartment, gazing out the window, watching the battle-ready soldiers rush through the streets, cutting down every undead creature that crossed their path. As the undead threat drained out of the city, survivors slowly crept back in to pick up the pieces of their fragmented lives.

Unwilling to believe the threat was finally gone, that they were no longer alone, Penny and the boys descended the stairs, fully armored, weapons in hand, and slipped unnoticed onto the streets. Sure enough, human beings were limping through the newborn sunlight, past the wrecked buildings and the rotting, festering corpses that had yet to be cleared away. No one was lunging for their throats. No one shot at them because they were scavanging for food in the wrong areas. People simply... lived.
Penny took a few tentative steps forward, gun limp at her side.

There was Mr. Marlow, the man who ran the wine shop down on 82nd, and his wife. Jessica and her interesting Salsa-dancing lover from 2a. That annoying man who liked to shout at her every time he passed her on his bike. All alive. Society would start to rebuild itself, she knew. It would take weeks before the city would start functioning at a fraction of its normality, months before people stopped being quarintined for sneezing, but the worst was over.

There was more hard work ahead of them. They no longer had just themselves to think of, and to answer to. Rules would once again come into play, life would yet again change durastically.

Long fingers twined around hers, and the grip on her hand was firm. She looked up to see Sheldon standing beside her, looking out at the street with grim determination.
They both ignored the sputtering disbelief resounding from the rest behind them.
Nothing would ever be the same. As they stood together and faced the dawn, they both silently accepted this in their own way, and were almost glad. Zombies ripped apart a large percentage of the population, governments were de-stablized and thrown into chaos, and - quietly - two people fell in love.

"I guess we should go help with the cleanup," Leonard appeared at their side, looking rather glum at his realization.
"Good idea, Leonard." Howard joined them, with a silent Raj in tow. "There are desperate, frightened women out there just waiting to be comforted."
Penny said nothing, just smiled and squeezed Sheldon's hand as the five of them squared off and faced the morning.