A/N: I was asked over on my tumblr what I thought about Bruce and Natasha's kiss in Age of Ultron. Personally, I liked it (although I agree with another fan who said they would have liked to see it from a better angle). I thought it fit with a number of the themes that ran throughout the movie for the relationship: the spontaneous, now-or-never aspect of it fit with how they'd talked about running with it and missing their window, and the context of it, Natasha using it as a diversion before she made Bruce transform, worked well for their shared struggle of identity and the roles they wanted to play vs the roles they had to play. That said, I'd be lying if I claimed I hadn't thought of what their first kiss might have been like under more ideal circumstances. This ask finally gave me an excuse to write it.

Bruce laughed, and it sounded too loud to his own ears. Not because it actually was; more like he'd become suddenly aware of a distinct lack of background noise to balance the volume. He blinked and looked around the dimly lit lounge, the detritus of the Avengers' farewell party everywhere he glanced.

"Um…" As he reached to scratch his head, his hand brushed against Natasha's, which rested on the back of the sofa. "How long have we been the only ones left at the party?"

"You know, for a scientist, you can be amazingly unobservant."

"Guilty as charged." He also hadn't noticed at what point in the evening they'd ended up on the sofa together, their bodies angled inward toward each other; the knees she'd tucked under herself just touched his thigh, her arm stretched behind him. "You probably kept a running count of who was here and their precise locations in the Tower at any given point all night."

"What kind of super spy would I be if I didn't?"

"A not so super one."

A dorky thing to say if ever there was one, but Natasha's smile belied her eye roll, and her husky voice replayed in his mind. Chicks dig that. Now she stared at him, a look of expectation on her face. Waiting for him to say something else, but Bruce racked his brain and came up blank. He swallowed. His throat was dry, but the beer he'd nursed was long since finished off, the bottle standing on the coffee table along with the dozens of others that littered the room, the wine glasses with dots of sticky red residue at the bottoms, tumblers with watery dregs of whisky. Bottles, bottles, everywhere, but not a drop to drink…

"What time is it?" He held out his left arm, pushed up the cuff of his sleeve–at some point, he'd shed his sport coat and draped it over the arm of the sofa–and looked at his watch. "Way past midnight."

"And your ball gown didn't turn into rags. Imagine that."

"I don't have. I've had entirely too much experience with that."

Now why did he have to go and bring the Other Guy where he wasn't invited?

But Natasha teased, "With ball gowns?"

He could have kissed her for it.

His heart stopped as he looked into her bright eyes. He could kiss her. She wanted him to, if he hadn't misinterpreted that whole scenario at the bar…or the whole rest of the night after that. If he had, then Steve had looked at the data and drawn the same wrong conclusion.

Turning more fully toward her, Bruce reached out, intending to cup her cheek in his hand, but at the last moment he second-guessed himself and brushed the tips of his fingers along the collar of her ivory satin jacket instead. (Was it a jacket, or a shrug? It had been more time than he wanted to count since he'd given thought to the finer points of women's fashion. For some reason, bolero stuck in his head, but that could have been completely wrong.)

"I'll leave the party dresses and dainty shoes to you, if you don't mind."

He noticed that the feet which peeked out from beneath her dress as she sat with her legs curled up beneath her were bare, heels discarded beneath the coffee table.

"Probably for the best," she said. "They're a bitch to run in."

Although the words were exactly what he expected from Natasha, the voice she uttered them in wasn't, quite. Not so much husky as usual as breathy, pitched slightly higher. Soft. Her skin was, too, and so pale in contrast with her black dress. As her hand must be, combing his hair back from his temple. Her chest rose and fell quickly beneath the clinging satin as his fingers skimmed along the ridge of her clavicle; Bruce felt the roll of her throat as his thumb found the hollow of her throat.

"You mentioned running, earlier," he murmured, leaning in closer, but still not giving in to the urge to kiss her. Was he savoring the moment, or simply too scared to actually make a move that would alter everything between them? He could smell traces of her perfume, applied hours ago, before the party. "But not away."

"With it," she said. "I asked you if I should run with it."

There was his out.

"Yes." He barely got the word out at a volume above a whisper, so he nodded for good measure.

Natasha touched her forehead to his, traced the line of his cheekbone with the backs of her curled fingers until her thumb brushed the edge of his lower lip, but stopped short of kissing him. Bruce had to laugh at himself, catching his lip between his teeth as he ducked his head. If he thought Natasha would ever let anyone off the hook, then he didn't know her at all. Didn't deserve to kiss her. Well, he didn't, but apparently he was the only one who thought so. You both deserve a win. If Captain America thought this was a good move, who was Bruce to argue?

"Bruce…"

And arguing with Natasha, he suspected, was as futile as engaging her in hand-to-hand combat.

He tilted his head, held her chin in trembling fingers–like a teenager who'd never done this before, which wasn't at all how a man his age wanted to feel–and touched his lips to hers.

Soft. They were so very soft, like her voice and her skin, and just like he'd imagined in the moments he'd allowed his thoughts to linger on her full lips. Nevertheless, he found the actual sensation of them against his surprising, as he did whenever he was confronted with this facet of Natasha's personality. That she was soft, gentle, careful seemed so at odds with her very name, the Black Widow she'd been created by the Red Room to be. When she first came to him in Kolkata, he'd mistrusted it, but now that he knew her, he had no doubt it was real, that this was who she truly was.

They went on like this for some time, light kisses and delicate caresses, hands embarking on a chaste exploration of necks, waists, hair, or twining together, clasped against his chest. All the while Natasha kept pace with him, achingly slow, never pushing for more than he was ready to give. It made him wonder how much experience she had with romance, apart from seduction. One of his hands rested in the curve of her hip; now he slid his arm more fully around her, drawing her body closer to his as he deepened the kiss. She opened her lips to him readily, with a soft sigh. Or maybe that was his own moan as her tongue brushed with his and her fingers hooked over the knot of his necktie.

At first he thought she meant to loosen it, or to unbutton his collar. Then he became aware that she was leaning back in his embrace, stretching out along the length of the sofa, using his tie to tug him down with her. All thoughts of restraint fled from him, then, and he did his best to shift from a sitting position to settle his body over hers without breaking their kiss.

He did break it, though.

They both did, at the sound of something else breaking, coming apart to snap their heads in the direction of the coffee table. In the process of moving, he'd kicked it, knocking a precariously placed wine glass onto the concrete floor.

Bruce cringed. To think he worried about how the Other Guy would act at parties.

He looked at the mess, then back up at Natasha. Her lips were pursed, twitching around at the edges in an obvious attempt not to laugh.

"Is this the part where I make a joke about how that kiss was really earth-shattering for me?" he said, tugging at his hair in back.

Natasha threw back her head, her laugh echoing in the big empty lounge. "Oh my God, you are such a dork!"

He caught his breath as she squirmed beneath him. The exposed curve of her neck was inviting. Irresistible.

Lowering his head once more, Bruce murmured against her pulse point, "A reliable source tells me chicks dig that."

He felt her abs tense against a shiver as he trailed soft kisses along her neck, to the sensitive spot where it met her ear. She wove her fingers through his hair, guided his head back up so that their lips were a breath apart.

"Or maybe it just takes one to know one," she said. "I was thinking more along the lines of that being a hell of a way to crash a party."

Bruce's groan might have started in response to the joke, but soon enough it was because she was kissing him again.

And because if there was one thing he regretted, it was that he hadn't noticed much sooner they were the only ones left at the party.