Chapter Text
So Peter had a hypothesis about it.
The first time it happened was the day after Homecoming, and honestly, it scared the shit out of him.
OK, so technically that wasn’t true. If you wanted to be all nit-picky about it, the first time was actually after the school trip to Oscorp. But that one didn’t count. Not “officially”, anyway. He usually used it as the control, the measure he could compare every other episode to, if only so he could say ‘this one’s bad, but at least it’s not as bad as that one was’.
The first first time, after Oscorp, the mutation lasted a little over two days. Ben and May told him afterwards that those were the most terrifying two days of their lives, but fortunately he spent them either completely blacked out, or delirious enough not to remember the worst parts.
The second time, he was conscious for each and every awful second.
At first, he thought it was just the aftermath of the battle with Toomes. Once he knew Happy had retrieved both the Vulture and the cargo that had survived the crash, Peter had limped back to the apartment, showered away the blood and sand and the smell of smoke, and then immediately passed out on his bed, vaguely aware of feeling a little...off. May had woken him before she left for work the next morning, asking if the plan was Disney or horror for the movie night later on, before pressing a hand worriedly against his forehead.
Half asleep, Peter had batted the hand away, saying he was fine May, really, I don’t need you to stay home, because he was, and he didn’t, because Spider-Man didn’t get sick. He hadn't had so much as a sniffle since the beginning.
An hour later, it started.
It was the spider bite all over again. The same nausea, some invisible hand grabbing hold of his stomach and clenching until he had heaved everything up, leaving him drained and gasping.
And then, the pain, the feeling of being burned alive from the inside out, starting at the base of his spine before spreading through his entire body. While his insides boiled, his skin froze over, every one of his senses jumping into overdrive, bombarding him with sounds and smells and blinding light and making him see things that weren't there, fire, and shrapnel, and icy waves threatening to pull him away.
He remembered how the two pinprick scars on his wrist had once again started to sting, remembered thinking through his fevered haze that maybe this was it. Maybe his powers were disappearing, maybe he had messed up too badly on the ferry. Maybe this was the world deciding that Spider-Man wasn’t needed anymore.
Mercifully, it only lasted four hours. By the time it was over, the injuries from the battle had nearly healed. More importantly, though, he was still Spider-Man. He hadn’t “un-mutated”, his powers hadn’t vanished. He went right back out on patrol that afternoon, and if anything he felt stronger, his senses more precise.
It had happened three more times since then, usually after some serious shit went down and he came out of it a little worse for wear. He'd be struck by that same burning agony, the same terror that this episode might be the one that took everything away - and then the same relief a few hours later when it was over, the pain stopping as suddenly as it had started.
OK, it sounded bad, but really, it was no big deal. May and Tony didn’t need to know, because it was nothing. Really.
Anyway. His hypothesis.
The spider bite had some awesome and some… not-so-awesome effects on his biology. You couldn’t have everything, after all. Sure, he could lift a school bus one-handed, he could hear a bank being robbed a mile away, but he also had problems with thermo-regulation (always a delight to deal with in the winter), and apparently he sometimes slept with his eyes open (which, after some extensive googling, he and Ned concluded must be something to do with the so very gross fact that spiders don’t have eyelids).
So yeah. The episodes were probably just one of the very -not-awesome effects of what happened, maybe the equivalent of when spiders shed their skin or something. It was just his body growing, his DNA mutating, bouncing back after he took a particularly bad hit. An inconvenient side-effect of being a friendly neighbourhood, spider-themed vigilante, with an undetermined amount of radiation bouncing round in his genetic code.
That was his theory, anyway.
As of yet, he’d never been able to confirm it, but it wasn’t as if he could go to a hospital and ask them to run some tests. And it was fine. It was really, honestly no big deal. He could handle it - and so far, thank god, he had pulled off the minor miracle of doing so without anybody else finding out.
But, as he already well knew, you couldn’t have everything. And, as he also already well knew, Tony Stark was an extremely difficult man to hide a secret from.
If there was one thing Tony hoped he’d be rewarded with after, y’know, saving the universe, it was a full night of sleep.
Apparently, though, preventing the violent destruction of only one reality fell just short of the mark for that.
“Boss?” F.R.I.D.A.Y said, for the third time.
“Yeah, m’up.”
With a groan, Tony rolled over and looked at his watch with bleary eyes.
God, it was early. Monstrously early. He hadn’t seen a time this early since Morgan started sleeping through the night.
“Sorry to wake you -”
“Yeah, sure you are.”
“- but Mr Parker seems to be in distress.”
Tony choked on a breath, suddenly very wide awake.
“The kid?”
“Yes, boss.”
He sat up, ignoring the post-battle ache that still lingered in his everywhere as he threw off the covers and got out of bed, stumbling in the dark and slapping the wall in search of the light switch.
“What’s wrong with him?”
“I am...unsure.”
He threw an accusatory glare at the ceiling. “I coded you to do better than unsure.”
“I'm running scans, but they haven't been able to determine the cause of Mr Parker's symptoms.”
In the back of his mind, he was vaguely aware of Pepper calling after him as he all but sprinted out of the door, but the panic was blocking out almost everything else.
He cursed, stumbling to a halt at the end of the hallway as he realised he had no idea how to get to Peter’s room. He knew his way around the recently re-bought Avengers Tower (he had designed the damn thing) but it was far too fucking early and the building currently housed so many people while they recovered from the battle with Thanos that he had somehow lost track of where the kid ended up.
“F.R.I?”
“Room 34C. Straight on, then to the right.”
He half-walked, half-ran the rest of the way, not bothering to knock when he reached Peter’s room.
He burst through the door, and a breath got trapped in his throat.
The kid wasn’t in his bed.
Shit. The kid wasn’t anywhere.
No. No no no. Not again.
“This better not be a prank, Parker.”
Spinning in a circle, he looked up at the ceiling (which, in the kid’s case, was always an important spot to check) and down under the bed, before he noticed a thin strip of light coming from inside the bathroom. He threw the door open, just managing to catch it before it hit the figure huddled on the ground inside.
Shit.
Peter was sat doubled over with his back against the tiled wall, both arms wrapped tightly around his midsection. His forehead was pressed against one bent knee, but even from the strange angle Tony could see the paleness of his skin, the pained grimace twisting his face.
“Kid?!”
Peter flinched, his whole body jerking.
“Pete - shit , what - what happened, what’s wrong?”
This wasn’t happening. This couldn’t be happening.
Tony knelt down and put a hand on Peter’s shoulder, drawing back slightly when he felt how hot the skin underneath his fingers was. The kid was burning up, heat radiating off him, making the air in the bathroom humid and stuffy.
This couldn’t be happening.
For a moment, Tony was frozen, unable to make sense of what he was seeing. Peter had come out of the battle with a few bumps and bruises, sure, but it was nothing his healing factor couldn’t handle. Tony had checked. He had checked, double checked, and then practically held the kid down while every medical professional he could wrangle for two minutes checked and double checked again.
How was this happening?
“Kid?”
Peter just groaned, hands clutching at the fabric of his shirt.
“F.R.I, get Banner. Now.”
“He’s already on his way.”
Peter let out a pained noise of protest, looking up at Tony with glazed eyes. “Don’t.”
Tony put a hand to Peter’s cheek, his fingers slipping on skin slick with sweat.
“Kid, what’s going on?”
“M’just - ”
Peter broke off with a gasp, curling even tighter into himself.
“Pete?!” This couldn’t be happening, this couldn’t be happening. “C’mon, help me out here, what happened?”
Peter lifted his head, pressing it against the wall and squeezing his eyes shut with a groan.
“Kid, talk to me!”
“M’sorry,” Peter mumbled. “Told F.R.I not to - ”
He cut off with a choked cry, doubling over again and slipping sideways down the wall.
Tony caught him, supporting him with both hands on his shoulders while he searched wildly for what was wrong, but there was nothing. No blood, no broken bones, just a fever Tony could tell was far too fucking high, limbs that trembled and flinched away from his touch.
“Pete, c’mon.” This couldn’t be happening, not again, this couldn’t be happening . “You gotta tell me what’s wrong.”
“It’ll stop - just - m-mutating, don’t -”
Peter struggled, pushing himself out of Tony’s grip. For a moment, Tony held on, terrified that if he let go Peter would crumble back into dust and all of this, everything they had been through, would have been for nothing.
But then Peter’s hand wrapped around the pipe under the sink, and the metal folded beneath his fingers like tissue paper.
“Christ,” Tony said, drawing back. “Yeah, OK, super strength, gotcha.”
“Sorry,” Peter said, his voice hoarse. “But...m’fine.”
“Kid, this is not fine. This is the furthest thing from fine.”
“It’s fine,” Peter said again, water dribbling down to his elbow as the pipe came apart in his fist. “Just...go, it’ll stop - ”
“You’re joking, right? I’m not just gonna - ”
Tony was cut off when the bathroom door opened. Bruce entered the room, squeezing his thankfully no longer so enormous frame (apparently the Hulk was taking it easy for a while, whatever that meant) into the limited space behind him.
“Tony?”
“Bruce, it’s - it’s the kid, I don’t know what’s wrong -”
Bruce knelt down beside them, gently pushing Tony out of the way and touching Peter’s forehead with his undamaged hand.
“Peter? Can you hear me?”
Peter cringed at Bruce's touch, but nodded stiffly.
“What happened? Are you hurt?”
“N-no.”
“Are you sick?”
Shaking his head, Peter moaned and curled into himself again, his breathing ragged and shallow.
“He doesn’t get sick,” Tony supplied.
Bruce turned round to look at him, brow furrowed.
“Doesn’t get sick? Are you sure?”
“Well, that’s what he told me. Something about a super-powered immune system.”
“Any injuries from the battle?”
“He - yeah, a few, but he’s enhanced, he heals faster than Rogers.”
Tony checked, he made sure. This couldn’t be happening.
Bruce clicked his tongue. “Uh...is he allergic to anything?”
“I don't - no, I don’t think so.”
“Who’s he been in contact with?”
“Uh, I guess - I don’t know, everybody? He was on search and rescue with Steve - ”
“Do you know if anything like this has happened before?”
“No, nothing - ”
“Did he come in contact with the stones?”
“He carried the gauntlet for a bit, before Thor got to it.”
“Do you know of anything else that might -”
“Christ, if I knew I wouldn’t be freaking out right now, would I?”
Tony hit his hand against the wall, his yell echoing hollowly off the tiles. Peter let out a pained groan.
Shit. He needed to get a grip.
“I don’t know,” Tony said, lowering his voice to a whisper. “I found him like this. He said something - uh, something about mutating.”
“Mutating,” Bruce muttered. His eyes widened in realisation. “Radioactive, right?”
Tony blinked, brain lagging a few seconds behind.
“Uh, yeah. Spider. Radioactive one.”
Bruce whirled back around, his voice suddenly filled with urgency.
“Pete? Peter?!” He gave Peter’s shoulder a firm shake. “Has this happened before, kid?”
Peter lifted his head. "Mm. Yeah."
“More than once? Since you got your powers?”
Another affirmative groan, and Bruce cursed, standing up off the floor.
“What?” Tony said. “What is it?”
Bruce didn’t reply, instead lifting Peter with one arm and carrying him out of the bathroom. Peter let out a sob as he was put down on the bed, the sheets getting twisted in his legs as he rolled over in discomfort.
Tony stood beside him, suddenly finding himself out of the loop and not liking it at all. He put his hands on Peter’s shoulders to distract himself from how they were shaking, watching Bruce root around in the bag he had brought with him.
“Spill. Now . Please.”
“I’ll explain later.” Bruce took a needle out of the bag and attached it to a syringe. “Hold him still.”
“Huh?”
“I need a blood sample, hold him still.”
Running mostly on autopilot, Tony allowed himself to follow the order. He grabbed hold of one of Peter’s arms and turned it over to expose the vein, holding firm as Peter tried to struggle away.
"Sorry, kid."
Another shudder tore through Peter’s body as Bruce inserted the needle into his skin. He gasped quietly, every muscle rigid and trembling.
“S-stop.”
“It’s alright,” Tony soothed, brushing away matted hair and very determinedly not looking at the syringe as it filled with blood. “It’ll be over in a sec.”
God, please let it be over in a sec.
After filling not one, but three damn syringes, Bruce moved away and put them into his bag before heading for the door.
“I’ll be back as soon as I can.”
“What?” Tony was torn, wanting to follow but also finding himself physically incapable of leaving the kid’s side. “Wait! Aren't there any painkillers in there, Miss Poppins?"
"If his metabolism is anything like Steve's, he'll just burn off anything we give him," Bruce said. "It'll be over in a few hours."
"What am I meant to do 'til then?!"
“Just, uh, stay with him.”
“And if he passes out or something?”
Bruce paused, tapping his fingers on the door frame.
“Yes. Passing out. Actively encourage that.”
And then he was gone. Tony turned back to Peter, who had stopped writhing and was now curled on his side, sweating and shaking and sucking in deep, laboured breaths.
“Mis’r Stark?”
“Yeah, kid, I’m here.”
“M’sorry,” Peter mumbled through gritted teeth. “I didn’t...didn’t want you to...”
“Hey, it’s OK. Just focus on me. You’re gonna be alright.”
Peter met his gaze for a moment before he allowed his face to crumple, tears spilling over.
“Hurts.”
“I know, I know,” Tony soothed, hoping his voice didn’t betray the fact he very much did not know, and in fact had absolutely no idea what was happening. “Bruce’ll fix it, it’ll be OK. I’m not losing you again, Parker. The world needs Spider-Man back.”
“You sh-shouldn’t...be here.”
“Nowhere I’d rather be.”
“But...Morgan.”
“She’s four years old, and it’s three in the morning. She’s off having fun in dreamland.”
Peter opened his mouth to reply, but then grimaced and drew in a ragged gasp. Tightening his arms around his chest, he squeezed his eyes shut against another wave of pain.
“It’s alright, Pete, breathe.”
“It really hurts, sir - I can’t - ”
“Yes, you can. You’re Spider-Man, you got this.”
“What if - I - god - ”
The kid jerked, rolling onto his back with a cry. One of his hands smacked against the wall, a red smudge marking the dent it left behind, blood beading up on his knuckles.
Tony gritted his teeth, bracing himself and engaging the gauntlet in his watch. He reached out and caught Peter’s hand as it flailed in the air, trying not to worry about it when he felt the nanobot layer give slightly under the kid’s grip.
“Want...May. Please. I need May.”
Tony swallowed. “We’ll find her, alright? You and me, as soon as this is over.”
“M’sorry. I’m sorry, sir, I’m sorry - ”
I don’t wanna go, please, sir.
I’m sorry.
“Nothing to be sorry for, kid,” Tony said, grateful Peter was too delirious to notice how those words took the air right out of him. “Just - just breathe, with me."
Peter’s eyes flickered up to meet his. “I c’n do it…’lone.”
“But you’re not alone." Tony gave Peter's hand a reassuring squeeze. “I’m not going anywhere. And neither are you.”
“Oh, I’m sorry, did I interrupt?”
It was fair to say Tony had a practised poker face, but he had to admit, this one was a challenge.
He left the room for ten minutes. Ten fucking minutes, after spending hours at Peter’s side, somehow managing not to freak out while the kid cried and writhed and then fell into what very much resembled a fucking coma, staying with him while he lay still and silent and pale as death.
But of course, it was in the ten minutes he was gone that Peter decided to return to the land of the living. He and Bruce looked up in surprise as he walked in, pausing the apparently very casual conversation they had been having in his absence.
“No, please, don’t mind me,” Tony said, leaning against the door frame. "Carry on."
“Tony -”
“Mr Stark -”
“You OK?” Tony asked, folding his arms and looking pointedly at Peter.
The kid bit his lip and nodded.
“Good. Then you can answer my question. What the fuck was that?”
Peter glanced nervously at Bruce, shifting and wincing in a way that might have made Tony feel a little guilty, were it not for the volatile concoction of confusion and exhaustion brewing inside him.
Bruce gave Peter a grim smile. “Do you wanna tell him, kid, or shall I?”
“I don’t care who does it,” Tony cut in. “But will one of you please explain to me what the fuck is going on because I can’t - I literally cannot cope with this.”
Peter looked at Bruce, desperation in his eyes.
“Yeah, OK,” Bruce sighed.
Tony sat himself down on the desk opposite Peter's bed, fixing them both with what he knew was his most piercing glare.
“So, uh...well, the good news is it’s not gonna happen again.”
Tony raised his eyebrows. “Well, that’s just fantastic. Oh, happy day. Shall we break out the champagne?”
“Tony.”
Bruce’s voice was quiet, but the warning in it was clear. Tony glanced over at the kid, noticing the dark circles under his eyes, the slight tension in his shoulders that suggested some of the pain still lingered.
“Right. OK...right, if that’s the good news, what’s the bad?”
Bruce’s brow furrowed, and he clasped his hands together. “I’ll, uh, try to put in English.”
“The kid’s a genius, Banner, you can talk science in front of him.”
Bruce laughed sheepishly. “Actually, I meant for you. Peter seems to have done his reading and this is, uh, our particular area of expertise.”
Tony blinked, looking between the two of them. “What, is there some sort of gamma-bros club I don’t know about?”
A weak smile flickered on the kid's face, and Bruce chuckled.
“Unfortunately not, but we should definitely start one of those," he said. "So, uh, you’re obviously aware of how Peter got his powers. His specific and frankly ridiculously lucky set of DNA, plus spider venom, plus a splash of radiation, and boom, you got yourself a vigilante.”
“Yeah, I’ve heard it from the vigilante himself,” Tony said.
Bruce hummed. “Did the vigilante himself tell you about the mutation period?”
Tony hesitated. The short answer was yes, Peter had told him. The honest answer was yes, Peter had let it slip very briefly and then got very uncomfortable, and Tony had, out of what he insisted was nothing more than professional curiosity, asked his aunt for the details. And then wished he hadn't.
“It came up,” he said finally, May’s account of impossibly high fevers and screaming in pain stirring in his memory. “But Peter’s all levelled-up now, right? Or are you due to grow another four limbs?”
Peter huffed a laugh, but Tony noticed the smile didn't quite reach his eyes. Normally, the kid would play along, quip right back at him, but right now he was tense, watching him intently but then refusing to make eye contact whenever Tony glanced his way.
Bruce sighed. “The thing about radiation is it’s...well, it’s unstable. That’s kind of its whole deal. The initial mutation period was the start of Peter’s powers, but it wasn’t the end.”
Tony suddenly started to feel a little ill.
“I went through a similar thing, after my accident. After the initial changes happened, there were several other, smaller mutation phases, usually triggered by a high stress event. The immune system goes into overdrive and tries to prepare the body for the next time something happens.”
Tony’s jaw clenched. “And I assume dying, then un-dying, then fighting in the biggest battle in human history would count as high-stress?”
Bruce nodded, eyes flicking momentarily to look at Peter. “We can assume so, yeah. The fact Peter came into prolonged contact with the six most radioactive objects in the universe probably didn’t help things, either. But don’t worry, I managed to develop a way to stabilise my DNA, stop it mutating any more than it already had. It should be quite easy to adapt it to match Peter’s genetics. It’ll need a few rounds of injections, plus a booster any time something...you know, high-stress, happens again, but we should be able to stop any more episodes from happening.”
“Just, uh, quick rewind,” Tony said, looking pointedly at Peter. “Exactly how many episodes have already happened?”
Peter looked down at his hands. “That was number five.”
There was a pause. Tony swallowed, took a breath, forcing down the reaction he wanted to come out with.
“Wanna expand on that?”
“Well, I guess it’s technically number six, if you count the initial phase,” Peter rambled, watching him nervously. “But I usually just...you know, ignore that. The time after the thing with the Vulture was the first smaller episode like this, and then the second was after I fought the Rhino, you know, the guy with the - ”
“Wait, wait, wait, just - hold on,” Tony said, holding up a hand. “This has been happening during the time you’ve known me? Why the hell didn’t you say anything?”
“I just...I don’t know. I didn’t think it was a big deal.”
“A big deal? Are you kidding me? I came in here and found you dying, Pete, how the hell can you - ”
“I wasn’t dying .”
“It sure fucking looked like it!”
“Look, I know I should have told someone, b ut I just...you know, I thought it was weird Spider-Man stuff that no-one needed to know about.”
“For fuck’s sake, kid! Does May know?”
“Of course not! I already worry her enough as it is, I wasn’t gonna give her another reason to - ”
“So what, you just crawl into a hidey-hole whenever this happens and ride it out alone?”
“Yes! Obviously!”
Bruce moved forwards, holding up a hand between them. “Maybe we should all just take a minute and - ”
“No, I’m not gonna take a minute,” Tony yelled. “I’ve taken more than enough minutes, stood here watching you, watching - watching that , happening, and not knowing what it was, or how to fix it - ”
“It’s fine, Mr Stark - ”
“Oh, that sounds familiar. You said that earlier, right, while you were dying?”
“Tony, that’s not what he - ”
“No, no, it’s fine!” Tony said, striding around Banner and yanking open the door. “You heard the kid. It’s fine. It’s all just fine.”
With that, he walked out and slammed the door shut behind him.
He needed sleep, and food. He needed to see his daughter, and though it tore him up inside, he needed to not think about this, not think about how Peter had cried in his arms, not think about his voice calling his name behind him as he walked away.
Shit. This wasn't how he wanted this to go.