Work Text:
Prologue
Everything smelled wrong. That was the first thought that filled his head when he woke up. Everything felt wrong too. His body, the air in his lungs, everything. He heard nothing to tell him where he was, what had happened...
They had been investigating an abandoned base, but he had had his guard up, of course, Hydra didn’t always leave without leaving some mementos behind, but everything had been cold, dead, smelling musty and long-abandoned.
But he had been unconscious and woke up feeling wrong. Almost without conscious thought his right hand flew to touch the left and...
His eyes snapped open when he felt warm flesh.
The first thing he saw was a shocked stare from blue eyes he knew even better than Steve's.
His own.
*
“Does the world really smell so different to omegas?”
“Well, apparently everything smells like a threat to alphas,” his own voice replied.
“Except of course... omegas.”
The frisson of... fear, had to be fear, in his stomach had to be a biological instinct, and nothing to do with him, person inhabiting the body. Stark obviously could make the alpha swagger work purely on instinct. Then again, he did swagger even as an omega.
There was shouting outside the lab, and now the uncomfortable feeling was definitely caused by alarm and... did all omegas react so strongly to alpha aggression? Purely physical reaction they couldn't stop? Because Bucky would never, ever, no matter the body he was in, be scared of Steve's blustering.
“Let the dumbass in,” he told Bruce, then frowned, because his voice was Stark's voice at the moment and Steve, for all his heart, could be a judgemental little shit about their second resident genius.
Like a magnet, Steve's eyes first sought Bucky's form, and Bucky rolled his eyes.
“Right here, jackass,” he said, knowing Steve had been briefed. And as much as Captain America would worry about any team mate, he knew what he really needed to hear first, every damn time.
Steve didn't know how to deal with him, obviously - he so clearly wanted to hug him, but alphas didn't in general go around hugging omegas that weren't theirs, and Bucky couldn't exactly blame him for it either. Not to mention, the body he was in wasn't just any omega, it was Stark's and...
...and Bucky had just slowly been trying to get to terms with being in his own body, this was a curve ball he didn't need.
Hydra had changed him, body and mind. They had trained his natural instincts out of him - they wanted the territorial alpha nature, but none of the protectiveness. Their Winter Soldier couldn't be swayed by omega distress, not even for a second. Their asset couldn't be distracted by the mating drive, so even an omega in heat would have to smell like danger to him.
When his instincts started to resurface after the regular brainwashing was over, Bucky didn't know how to trust them. Alphas naturally viewed half the world as a threat, and he remembers fighting with that all his life before the Chair, but omegas... Omegas had started to smell good again, tempting, and he couldn't trust that either, because he couldn't remember how to deal with that. And Stark, Stark had smelled especially good, but was that just his instincts going haywire, and him being the only omega around regularly, or was it something more?
Could it be? Should it be? And what did "something more" even look like in the future?
And then now, this. He had never fully appreciated how different the world seemed to omegas. And Stark still smelled good to him, which meant... Bucky smelled good to him, and for omegas, alphas smell threatening, and for a good reason, except for those who... No.
He had more pressing worries right now.
“Does it really feel this uncomfortable to be all the time?” Bucky asked.
“Uncomfortable how?” Bruce asked, frowning. It probably was different for a born omega, after all.
“Hot, cold, itchy, like my skin doesn’t literally fit – I'd think it’s the swap but Stark seems more comfortable in my skin.” There was no accusation in his voice. And if he sounded petulant it had to be Stark's voice's fault.
“Oh, that’s just the heat coming on,” Stark said casually and what?
“What?” The panic in the voice that definitely wasn't his really had to be another of those biological reactions he couldn’t help.
“What’s the matter? You just feel incredibly horny for a few days, and that’s it. If we're still swapped, we'll leave you in a room with a selection of toys and you’ll be fine.”
What was the matter with the man? He sounded so...
“Oh for fuck's sake, have ever seen a heat outside a porn movie?”
Bucky had never seen Tony Stark blush so the heat on his cheeks probably didn't show. Hopefully didn't show.
“To be fair,” Steve said, “back in the day you never experienced heat with a partner unless you were married.”
Nice omegas wait for marriage, even when alphas try to make them change their mind. Nice omegas lock themselves in as to not be a temptation, because the alphas can't be trusted. Nice omegas never walk alone, because every alpha is a threat. Nice omegas...
“Okay, so what’s the nightmare scenario going through your devilishly handsome head at the moment? Insatiable lust that makes you forget to eat or drink? Incapability of saying no to any alpha who offers? Self-harming unless you get knotted?”
“It's not...?”
“Which one? Well, it isn’t any of them. Half of the myths exist to justify alphas raping omegas – we do know how to say no, and do so, but historically, haven't been believed much. And, yeah, get horny enough and any alpha starts to look like a viable option. But even if you're starving, you don’t dive into a dumpster if there is a restaurant next door. That said, we are locking you in a room, because I don’t know what your self control is like, and if you get my body knocked up, we are not switching back until that is taken care of.”
Even Steve frowned at that. “Really? Surely you’re too old to have to worry about that?”
Bucky winced, because you never told an omega they looked old, but Stark merely scoffed.
“Are you kidding? My mom was 50 when she had me.”
“No one is getting knocked up,” Bruce the Voice of Reason said. “And Bucky, if you find the prospect too unnerving, we can try blockers. It’s a bit late, but...”
Stark was shaking his head.
“They don’t work that well on my body. Don't you think I’d be on them otherwise? I hate losing three days of working time.”
Stark? Resenting three days of nothing but sex? This wasn't what he’d expected, based on gossip rags and Steve's... Steve's judgmental assholery. Jesus, maybe he should learn to make his own mind about people even in the future because his best friend was an idiot.
“Idiot,” he said, and smacked Steve’s head. He had to reach further up than he was used to.
“What? What did I do?”
*
The oncoming heat was an issue, yes, and he wasn’t looking forward to it at all, but that wasn’t the only issue. (Well, the Hydra machine that had somehow made them switch bodies with Stark was a big one, but Bruce was working on that.) No, the biggest issue was that... Stark really smelled very good to him. Which meant... His body, the body Stark was inhabiting at the moment, smelled good for Stark. In heat. And maybe out of it. And he knew it might have been another myth but...
“Why didn't you ever tell me we comp?”
At least Stark didn’t try to pretend to misunderstand him. “What was I supposed to do? When Steve brought you in, reply to his introduction that you’d make a good babydaddy for me? That’s the only thing compatibility means.”
“It’s not." Maybe he was old-fashioned, a relic of the war, but mates was more than that.
“Look, I refuse to be tied up to anyone based on a stupid quirk of biology,” Stark said, and that... that Bucky understood.
“But surely it’s easier to fall in love with a comp match?”
“I should have known, you’re a romantic.”
“And what’s wrong with that?” Bucky demanded.
“Just don’t infect my body with feelings. Now! Something way more important.”
Stark hit his hands together in fake excitement, then glanced down, obviously distracted by the feel of meat hand meeting the metal. He shook his head almost dismissively before looking back up at Bucky.
“Why haven’t you told me the arm hurts so much to use?”
Bucky shrugged. What was the point? He was stuck with it regardless – being without it would be worse. Much worse.
“I’m sorry it's causing you pain,” was the only thing he said. “A sling will help.”
Stark was busy flexing the arm, seemingly testing the reactions of the fingers, and part of Bucky wanted to yell at him to stop. It felt wrong to be without the arm, wrong to feel two arms the same way, and he always made too wide movements with the left now, because it was too light.
“Boring. I'm sure we can improve it.”
“No! Don’t you dare... Stark, I mean it! I will shave your goatee and dye your hair pink!”
“Pfft, I'd pull it off,” Stark merely said but then something on his face must have gone through to the other man because his face softened.
“I would never make any physical changes to your body without your explicit prior consent,” he said, and it was obviously a solemn vow. Bucky touched the borrowed sternum he was wearing, feeling the unnatural stiffness of the metal, and swallowed. Yeah.
Wait, the smell... he groaned. Stark didn’t need to read his face. The room was so filled with “omega in distress!” that he wondered Steve hadn’t barged in with the cavalry.
Still... he trusted Stark. His presence wasn't aggravating it. And he believed his promise. Would it be so bad...? Stark had made the joints stick less. Maybe...?
“Scans,” he said. “Maybe just start with scans?”
*
Maybe they should have remembered the on-coming heat before they locked themselves into Stark's workroom alone.
First “Stark" became Tony, because when a man was so familiar with your body, it was hard to keep up barriers.
Then “Tony” became something he was fast starting to think as “his" which was a weird flash of alpha possessiveness in an omega body... did omegas get as possessive of their alphas as well? Just the words “my alpha" seemed to awake that frisson Bucky was fast realising wasn’t really fear.
But was all this, all these feelings, just... biology? He didn’t know what it felt like being an omega, not really, he hadn't grown up into it.
“Is this an omega thing?” He asked, when once again he had unconsciously wandered over to stand close to Tony, almost to touching distance, and the only one he had been comfortable doing that had been Steve, and even he only on a good day. Steve was an alpha, and dark instincts Bucky was hoping he could blame on the Winter Soldier programming instead of pure alpha-ness, were calling him an intruder in Bucky's space.
“Is what an omega thing?” Tony asked, moving smoothly around him, as if already compensating for his proximity, like it was a known and expected thing.
“This... closeness.”
“Maybe. Maybe that's why I carried it into this alpha body.” Tony's admission came with a side glance Bucky was tempted to class as flirtatious.
And he may or may not have smouldered right back.
“If you look at me like that,” Tony said without looking up from the work again, “you will get locked for the heat. But I'll be on this side of the lock.”
“But that's it, I seem to think that’s an excellent idea at the moment!”
“And you don’t know if that’s just the pheromones talking.”
Bucky shook his head while looking down.
“So I’ll go,” Tony said, like it was that simple, as if Bucky couldn’t recognise a dozen signs of desire on him, on his own body, so clearly it was like he could feel them himself, the need to bite down and claim... And surely that couldn't be an omega biological drive?
He fought back a whine.
"Or I stay for a bit, and you get into the Iron Man armor. It's scent proof."
"Really?"
"I'm an omega, of course my damn battle suit is scent proof. But that's not the point. I want you to feel safe."
"I feel safe with you," Bucky had said before he even knew he was going to open his mouth.
"Now I know that's the instincts speaking. And you can't trust them right now. So I'll go."
“I think we’ve got it!” Bruce's voice on the intercom was excited, and Bucky swallowed another whine.
“We, uhh, can't come up right now. Well, I'll be there, but Bucky is... indisposed.”
“Why, what's... oh. Oh! Yeah, all right. We'll do this in a day or two... are you...”
“I'm coming up,” Tony said decisively.
“Feel free to explore that body,” he said with a grin that looked surprisingly familiar on the face so like – but never quite – his own. The expressions were never quite the way he remembered them, “ You’ll be pleasuring it again soon enough.”
Now he definitely whined.
"But what it if all goes away when we turn back?" he had to ask.
Tony smiled, a smile Bucky hadn't even known his own face muscles could form, a benign, promising, gentle, almost sweet smile.
"Trust the 'omega thing'," he said. "I could smell you just fine before the swap. And compatibility may not mean anything beyond biological compatibility. But there's never been a proper mating without it either."
Epilogue
“Wait! Do you have anything?”
“I have a lot of things,” Tony promised with a leer.
“Contraceptives!”
“Don't you know anything, Jon Snow? It’s virtually impossible to get pregnant outside a heat. That’s what the heat is for.”
“So when's your next heat then?” Bucky asked, with a cocky smirk again feeling comfortable on his face. (His face! His own face!) Because as much as he'd feared the prospect on the other side of the equation? He was kinda eager to explore it from this side.
“When you're looking at me like that – and flooding the place with pheromones? Much, much faster than you’d think.”
“Can’t wait,” Bucky said huskily, and picked his omega - his mate - up for another kiss.