It amuses me that it's been pointed out how this concept is very fangirlish. Because it is, and I did it anyway. I am only slightly sorry. As for which Supernatural episode this was inspired by (coughstolenfromcough), I'm not telling. That would make this too obvious. Aside from the concept, the episode itself was not one I really liked anyway, so no great need to rush out and Netflix this sucker.

Warnings: one spoiler for the movie, but I'm expecting most people have seen the movie by this point.

It had been a week, and no one knew quite how to deal with the intruder in their home.

This was how Tony Stark was currently defined. Intruder. He was not, in fact, Tony Stark but merely his likeness with a mind not his own residing in his body.

"It's like the goddamned curse of the body snatchers," Clint had said rather aptly one day.

One of the main reasons Bruce disliked magic was because it was so untested. It was like theoretical science that no one tried because very few believed it existed.

Well, it did exist, and he had already witnessed it. Thor was a borderline magical being in and of himself. The Tesseract had been a meld of science and magic that had nearly brought about the destruction of humanity.

Bruce wished Tony was around so they could expound on their mutual dislike for all things magical.

According to SHIELD files, there was one known entity who was familiar with the magical side of the spectrum. The only problem was, Dr. Strange liked to make himself extremely difficult to locate. It was not as though they had him on speed dial. Even more recent SHIELD files declared his location as Unknown.

Which meant they had to convince the person taking up residence in Tony Stark's body to tell them what had been done to cause this horrible allergic reaction.

Wait. That was just what Bruce kind of felt whenever he set eyes on the creature. It was difficult to consider it anything remotely human when it had overrun Tony's consciousness.

When the following week rolled around without any further headway made toward finding Strange, Bruce took a chance.

Thus far, only Pepper or Natasha had been remotely welcome in that room. Clint had been the last man to go in, and the person who was not Tony had promptly locked himself in the bathroom and refused to come out again until Clint left.

This meant Bruce had to be very cautious in how he proceeded. Because he had an inkling now as to what this behavior meant. Unfortunately for the intruder, he had very little sympathy for anyone who kept him from the people he cared about.

Not-Tony looked up from where he lounged on the bed, reading the copy of Breaking Dawn Natasha had produced. Bruce stared at the book and wondered how horrified Tony would be to know he had been seen reading it.

At least he was not in public.

Bruce pulled a chair through the door and set it against the wall, closing the door behind him. He could see the man—not-Tony—already tensing to flee, so he held up a staying hand. Surprisingly, not-Tony stilled, watching the proceedings very cautiously.

"I'll make a deal with you," Bruce said, and he was rather proud of how even his voice was considering how exhausted he felt. This was probably not a wise move, coming to visit this person, but he had made his bed. "I'll sit in this chair and not move from it, and you don't blockade yourself in the closet or anything equally ridiculous."

The expression he got for his trouble was one of poorly veiled revulsion. Bruce had to wonder what kind of men not-Tony had been around to elicit that kind of reaction.

He sat in his chair, as promised, crossing his legs and resting folded hands atop them.

"What should I call you?" he asked to start. "Because you're obviously not Tony."

"Close enough," the intruder murmured silkily. Bruce felt an unhappy tremor rush up his spine, and he closed his eyes to distance himself from the sight of Tony combined with that deliberately provocative drawl.

"No, it's not," he said when he had collected himself. He fixed a harsh glare on the intruder. "You don't seem to understand the position I'm in."

"I understand that you people are keeping me in a fancy jail cell," not-Tony sneered.

"You understand nothing!" Bruce snapped. "You are a child playing a game with people who are way out of your league. Do you even know whose body it is that you are using as your own personal dummy?"

"Tony Stark," the intruder said snidely. "I hear it plenty."

Bruce took a bracing breath.

"How about the rest of us? Do you know who we are?"

"Why would I want to know?" the intruder snarled. "You're a bunch of assholes. What else do I need to know?"

Bruce stared at the intruder, taking in the three-day growth of stubble and the unfriendly curl of his lips in it. (Unwilling to let Tony go to total scruff, Natasha had pulled out an electric razor and helped this idiot shave the beard down to nothing. Tony would have hated it.)

"You might be interested to know that not all of us are out to hurt you," Bruce said mildly. "If you had chosen any stranger on the street other than Tony Stark, I might even be inclined to let you continue with this bizarre charade."

"There's nothing you can do about it," not-Tony said, though he looked highly uncertain of this fact. "I'm here now. Your friend is gone."

"Is he?" Bruce gazed at the strange person wearing Tony's face. There was a darkness building, malice bubbling up that was nothing like the other guy's rage. This was not anger. It was hatred. "If that's true, then you have a problem. Because Tony is one of only two people in this world that I would do absolutely anything for, and you're taking that away from me."

"Oh, boo-fucking-hoo."

"Your problem," Bruce continued, plowing over the snide retort. "Is that you don't know who I am and what I'm capable of. You're making enemies of people who can do very bad things to you without fear of retribution. If we chose to hurt you, there's absolutely nothing you could do to stop us."

Bruce would have relished the fearful look he received if it had not been on that face. Intellectually, he understood that this was not Tony. Unfortunately, his eyes were telling him a different story. Tony had never turned that angry fear on him before, and it hurt something in Bruce to see it now.

The hatred roiled beneath the surface. He should not be in this room. He could not be in it much longer.

"There is nothing I would not do to get Tony back," he said bluntly. "I'm going to make a one-time offer now. You give him back to us, and we'll forget this ever happened. We'll leave you alone."

The fear did not dissipate. It remained solid on not-Tony's face, mingling with uncertainty and finally leveling out into sheer stubbornness. Bruce stood.

"I'm through with you," he declared. "Enjoy the rest of your life rotting in this room because you're not going anywhere else."

"You can't do that!" Bruce did not have any difficulty discerning this voice from his friend's in its protest. It was too high, too panicked, borderline shrill. The sound was nothing Tony would have ever made. "I have rights!"

"No, you really don't."

He flung the chair out into the hall and slammed the door behind him.

Steve was there, looking sad and stern all at once. Bruce did not want to hear it.

"I'm fine," he growled.

"You're not," Steve said, quite correctly. "Fortunately, there's something by the Hudson that needs our kind of attention. You up for it?"

"I think I could stand to wreak some havoc."

Steve's smile was surprisingly vicious. It seemed that everyone was feeling the stress lately.

The intruder's name was Cassie. It might have been a name chosen out of the blue, but Bruce had added it to the file he had amassed in his attempts at identifying this thing that had taken over Stark's body.

Natasha had become the confidant, though she was liking the game less and less as time passed. (In truth, she had never enjoyed it, but it was getting painful.)

"If I have to watch one more episode of Vampire Diaries, I'm going to commit homicide," she had muttered over her coffee that morning.

She was in luck that day. They were onto the first episode of Charmed.

Of all of them, Natasha had the least amount of difficulty identifying this person with Stark's face under a different name. While even Bruce struggled to consider the intruder anything more than a doppelganger, Natasha had taken to calling the man Cassie with ease.

It helped that she was assisting him with all the masculine grooming that no one else could get close enough to do. It also helped knowing that the intruder was getting more thorough in his shaving habits than Stark ever would. Natasha was pretty sure he had shaved his legs and under his arms. That, and he liked to experiment with more gymnastic forms of exercise. Natasha was teaching him balance, and for some reason he enjoyed handstands. It was strange and disturbing on all sorts of levels.

"Am I really going to be stuck here forever?" the intruder—Cassie—asked that afternoon when Natasha stopped in with dinner.

Natasha stared at him, level for once.

"Yes."

Cassie looked at her, startled by the calm declaration of truth. Natasha sat cross-legged on the bed and considered the man next to her. Cassie had dressed once again in jeans and an oversized sweatshirt, the sloppy androgynous look telling. Clint had nailed it long before Natasha got a name out of the intruder—this person was used to being a girl.

"You chose a high-profile man to hold hostage," Natasha added.

"I'm the hostage!" Cassie protested.

"As far as the others are concerned, you've committed a crime," Natasha said coolly. "This is either kidnapping or murder. You're lucky they chose not to put you in a federal prison. With looks like Stark's, you'd be someone's bitch in no time."

Actually, that was assuming no one ripped the arc reactor out of his chest. Then he'd just be dead. Cassie was ill-equipped to protect that body. It was for that reason the Avengers were doing it.

Cassie paled and drew his knees to his chest, hugging them tightly. His eyes had dropped. Natasha knew exactly where his mind had gone.

"Who hurt you, Cassie?" she asked quietly. Startled eyes darted up, but Natasha would not let this go. She strongly suspected this was the reasoning behind everything they were dealing with right now. "I know what it's like. It was a long time ago for me, but for you not that much time has passed, has it?"

Apparently three weeks was not enough time to build a rapport strong enough to get an answer to that question. Natasha sighed when the man shut down and avoided her eyes.

She stood, gathered up the empty tray, and left the room.

Steve Rogers considered himself a reasonably intelligent man. He admitted he was nowhere near Tony's level of intellect, or Bruce's. Actually, Pepper was probably smarter than he was, but none of that made Steve stupid. He was a good leader and a great strategist, and he was aware of this.

However, he never had anticipated the fallout that would result from the loss of just one member of their team.

It was not the loss of one that had caused this. Rather, it was the loss of Tony which was creating stress fractures all along their team walls. He was, almost literally, the glue which held this team together. Steve had always known that, without Tony, Bruce would have long ago run away to some remote place. What he had not realized was that Clint was also ready to jump ship.

"What do you mean: you've got a mission?" Steve heard himself ask blankly that afternoon. "You haven't taken missions from SHIELD in months!"

"There's no one else," Clint shrugged, tossing a few shirts into his duffel. "It's not like we're doing anything here. I feel like a glorified prison guard. I need a break, okay?"

"We're going to figure out what happened to him," Steve said. After over four weeks, it was getting repetitive and weak. They had not discovered anything new since learning the intruder's name, and no one could find a consultant versed enough on magic to figure out what had happened. SHEILD was being amazingly unhelpful, which was probably Steve's fault for having shoved Fury's orders in his face as he had.

"You've got my number, Cap," Clint flung the bag over his shoulder and hefted the case which stored his bow and quiver.

The next day, Thor declared he had a great desire to visit his beloved Jane in New Mexico.

Two days later, Steve was ready to drop to his knees and beg Bruce not to go. It was a near thing.

"You look a little desperate, Steve," Bruce remarked. Steve had caught him on the ground level of Avengers Tower. He was dressed to go out, umbrella in hand as it was raining heavily.

"We're falling apart, Bruce," Steve admitted. "It's only a matter of time before Natasha gets sick of dealing with this too. Please, Bruce. Even Pepper has stopped coming every day."

"I'm just going to pick up some groceries," Bruce said, smiling wearily.

Steve knew the man was not leaving yet. He wasn't even packed. But he had to make this point at some time. Better to make a fool of himself now than to be begging when Bruce really was packing up to leave.

"Can I come with you?" he asked.

"Need to get out for a while, do you?" Bruce chuckled, but it held little of his typical wry humor. He sounded flat.

"Desperately," Steve admitted, fetching his jacket from the hook against the far wall. "I don't think I can take much more of this, Bruce. I always knew Tony contributed a lot to this team, but I never realized just how much."

Bruce sighed and glanced out the glass doors into the cold winter drizzle. It probably felt as miserable as it looked, but Steve was willing to brave the cold to get out of this stifling environment, if only for a short while.

"Part of the problem is that he's not actually gone," Bruce admitted. "We see him every day, but it's not him. It's a stranger, but because of that we've put our friend under house arrest."

"And you?" Steve asked warily. "You haven't talked to him in almost three weeks."

"I'm putting the female pronoun in front of that one," Bruce snorted. "No matter what Clint says, gay is not equivalent to teenage female."

"Isn't Clint gay?" Steve asked before he could stop himself. That was insensitive, and probably none of his business.

"Bisexual, more likely," Bruce watched him lace up his boots. "He's been pretty mellow on the dating front since Agent Coulson died. Tony might know. He's the only one who would ask."

"Thor might," Steve mused.

"Clint doesn't take Thor seriously off the battlefield. Tony is the only one Clint might answer."

That much was true. Steve stood and followed Bruce into the rain. It was as cold as he thought it would be, but for once he did not mind.