(A/N: The last few chapters were a little rough because I wrote them like two years ago, but this chapter is almost entirely new, so hopefully it's an improvement! Enjoy, and let me know what ya think!) 

"Tony Stark did what?!"

Natasha shifted in her chair as Steve Rogers and Sam Wilson gaped at her from across the table.

"I swear, he was robbing the Potions room," Bucky said from beside of Natasha. "We saw it with our own eyes."

"It's the truth," Natasha murmured through gritted teeth. She had pleaded with Bucky not to tell anyone what they had seen the night before, but the pig-headed git had insisted. Now, she found herself sitting with the trio of unruly boys in the corner of the Library, garnering strange looks from her fellow Slytherins.

"Why in Merlin's name would he do something like that?" said Sam, his brow furrowed.

"I'm gonna take a wild guess and say he's making a potion," Natasha replied in a withering tone.

"I'm certain his father could get him any materials he needed," Bucky remarked. "Why does he need to steal them?"

"Unless he doesn't want his father to know about what he's doing," Natasha argued.

"Which means it's something dangerous," Steve added with a frown. All eyes turned to the skinny boy as he drummed his fingers along the wooden table.

"Why do I get the feeling you're about to suggest a terrible idea?" Bucky muttered.

"We need to know what Stark's up to," Steve responded, his eyes blazing.

"And how do you plan on uncovering such valuable information?" Natasha asked, raising her eyebrows.

"I'll ask," Steve said, shrugging his shoulders. There was a pause, then Bucky broke the silence with a groan.

"Brilliant," Sam replied, clapping Steve on the back. "I'll start the funeral preparations."

"I'll threaten him if I have to," Steve said, ignoring Sam's jab as he stood to his feet.

"Steve, let's talk about this," Bucky said, grabbing his arm to prevent the boy from walking away.

"There's nothing to discuss, Bucky," was Steve's sharp response. "Stark could very well be threatening the safety of the school." Natasha sighed as she watched the blonde-haired boy yank his arm away from Bucky's grip.

"Let me do it," she interjected, and the boys' gazes swiveled back towards her. "I can get to him. Go undercover. I happen to be very... persuasive."

"Why are you here again?" Sam retorted in a scornful tone.

"And why should we trust you?" Steve agreed. Natasha held back a scowl. She looked to Bucky for some support, but was greeted by a blank expression.

"Well, if you're so intent on ending up in the infirmary," she scowled, "I've never been one to turn down free entertainment."

"Good," Steve replied, his eyes narrowing. "Now, let's get on with it."

******

Steve marched to the opposite end of the library, where Tony was lounging with his feet propped upon a wooden table and a worn leather notebook in hand.

"Sorry Rogers," Tony called, lowering his notebook. "The intense glowering class was moved to the fourth floor."

"Enough with the games," growled Steve, crossing his arms over his thin chest. "We know you were in the Potions room past curfew last night."

"Do you, now?" Tony responded, tilting his head to the side with a feigned look of interest.

"What did you steal?" Steve asked in a threatening tone. "What are you planning?" Tony laughed.

"I don't have to tell you anything," he retorted.

"We're not leaving until you do," Bucky piped up from beside of Steve.

"I guess you better get comfortable then," said Tony. Steve unfolded his arms and gripped the edge of the table, leaning down until he was face to face with the boy.

"I can do this all day."

Tony laughed.

"God, please tell me that isn't your new catchphrase." Steve shut his eyes for a moment, resisting the urge to punch the boy across the face. He knew it wouldn't make much of a mark anyway.

"Stark," Natasha broke in, pressing herself between Steve and Bucky. "Just tell us what's going on."

"He's making a super-strength potion."

All eyes turned towards the short, curly-haired boy standing just behind Sam. Steve narrowed his eyes. The boy shoved his wire-rimmed glass up the bridge of his nose, shuffling forward to join the crowd gathered around the table. "I helped him. I know what he's doing."

"Who is this?" Sam and Bucky asked in unison.

"Bruce Banner," the boy answered. "Ravenclaw."

"He's a prefect," Natasha added, as if in warning. Bruce sighed.

"I'm not gonna turn you guys in," he said. "As long as you let me help."

"Whoa," Tony interjected, holding up his hands in protest. "I don't need any help."

"And who said we're letting him do this in the first place?" said Sam. Tony snorted.

"You won't stand in the way of innovation, will you?"

"No, but I'll stand in the way of stupid."

"Says the guy with an IQ lower than his age."

"Hey -"

"Everyone shut up!"

Steve blinked, surprised when everyone listened. He cleared his throat, searching for words.

"Well?" Tony queried,

Steve stared into the boy's dark, stormy eyes. He could sense that Stark wasn't asking permission, but still there was a hopeful glimmer hiding behind his iron demeanor. As the seconds ticked by, a tense silence growing among the crowd, Steve arrived at his decision.

"Let me be the test subject," he said in a low tone. No one said anything for a moment. Then a chorus of voices all started shouting at once.

"You can't be serious!"

"Have you gone mental?"

"What would your mum say?"

"I'm not planning your funeral -"

Tony stood to his feet, dropping the notebook onto the table with a silencing thud.

"Ok."

He reached out a hand. Steve hesitated, then grasped Tony's hand, giving it a firm shake.

"No one else is in danger, right?" he asked, shoving his hands into his pockets. Tony smirked.

"No one except you, Rogers."

"We've got a deal then." Steve looked sideways towards Bucky, who responded with a murderous scowl.

"You can't do this, Steve," he hissed.

"I have to," Steve muttered back. "He's gonna do it anyway, and this means that no one else will be in danger."

"Alright," Tony interrupted, rubbing his stomach. "Spare me the speech before I throw up my lunch." He started packing his things. "Meet me in the Potions room at midnight. No one else besides you and your... gaggle of friends here. And Bruce, you might as well tag along too." Steve glanced around the table as everyone nodded in agreement.

"Then I guess we'll see you there."

******

It was a quiet night at Hogwarts as Tony made his way down to the Potions room. He shivered from the cold, wishing he'd brought a hoodie. Instead, he shouldered his backpack and made the last turn before sliding into the classroom.

Steve was already there, accompanied by Sam and Bucky. A spike of adrenaline coursed through Tony's veins. He swallowed the lump in his throat, throwing his backpack down on a desk near the center of the room.

"Ready to change your life, Rogers?"

Steve seemed to shudder for a moment, but he collected himself soon enough and joined Tony by the nearest cauldron. Tony directed Sam and Bucky to move some of the desks around to give them ample space. While the two of them set to work, Tony began unloading the jars of ingredients.

It was then that Bruce arrived, dusting off his cardigan as he approached.

"Banner," Steve murmured, dipping his head in a polite greeting.

"Bruce, give me a hand with these lacewing flies." Tony handed the boy a jar, then a mortar and pestle.

"When will the potion wear off?" Steve asked.

"Assuming my calculations are correct," Tony murmured, "Never."

When Steve didn't respond, Tony looked up from his backpack to see a look of shock etched across his face. Bruce wore a similar expression.

"H-how do you..." Bruce stammered. "What are you using to..."

"See for yourself," Tony said. He withdrew the notebook from his backpack, then set it on the desk. Bruce leaned over the notebook, adjusting his glasses as he scanned the pages, mumbling to himself.

"I... uh..." Bruce looked up, glancing between Tony and Steve. "I think this could work." Tony smiled, a burst of relief flowing through him. He would never have admitted it aloud, but he was glad Bruce seemed confident in the potion.

"So let me get this straight," Steve said, running a hand through his hair. "If this goes horribly wrong, and I end up turning into a... a gorilla or something... it's permanent?" Tony exchanged a look with Bruce, who shrugged.

"More or less," he replied. Steve narrowed his eyes, stepping closer to him. Tony tightened his grip on the jar of crushed leeches in his hand, standing his ground.

"You sure you know what you're doing, Stark?" Tony stared back into the boy's hardened eyes.

"Positive."

They stood there for a moment. Tony waited. Steve's gaze pierced into his own, sizing him up from the very depths of his soul.

"Let's do this then," the boy said at last. Tony released the air trapped in his lungs.

Before he could muster a response, Natasha burst into the room, breathless and frazzled.

"They... coming... couldn't stop..."

"What?" Sam screeched.

"Who?" Bucky and Bruce asked in unison.

"Steve Rogers!"

Tony froze as Peggy Carter stormed into the classroom. Her robes flew behind her as she stomped up to Steve and grabbed him by the scruff of his neck. She dragged him off to the far corner, where they began speaking in hushed, but heated tones.

"Romanoff!" Bucky hissed, drawing Tony's attention back to the redhead. "What in the bloody blazes happened?" Natasha gestured towards the door, still struggling to catch her breath.

"They... they were on patrol... caught me on the way down..."

"They?" Tony repeated. "Who else is -"

"No one told me we had a party going on in here!"

Clint Barton strode through the doorway, grinning like an idiot. The boy shoved his hands into the pockets of his Hufflepuff robes, eyes swinging around the room with a jovial glimmer. "Well?" he said. "Where's this experiment?"

"What did you do?" Sam asked, shaking his head at Natasha.

"I had no choice!" she growled. "If I didn't let them come with me, they'd have turned us over to Murdock."

Tony sighed, throwing his hands into the air in exasperation.

"Why don't we just invite Odin down here to watch!" he exclaimed, turning back to the cauldron with a resigned grunt.

With Bruce at his side, Tony set his mind towards adding the ingredients to the giant pot. The mixture began to bubble and boil, just as expected. Tony added the final dash of powdered lacewing flies, smiling to himself as the potion turned a dark blue.

A flurry of voices came from behind him, and Tony spun around to see Steve and Peggy walking towards the group that was gathering around the cauldron. Neither of them seemed happy. Tony found himself holding his breath.

"I will have you know," Peggy said, stepping up to Tony with a scowl that could have melted stone. "If this goes wonky, I'll have your head on Odin's desk before you can even breath." Tony smirked.

"You got a real keeper here, Rogers," he called over Peggy's shoulder. Her expression darkened.

"I'm watching you, Stark," she scowled. The girl backed away, taking up a position behind Steve.

"Alright then," Tony said, clapping his hands together. "The potion should be ready in about an hour."

"So now we wait," said Steve. The boy had dawned a grave expression, as if he were facing doom.

With nothing else to do, the group dispersed across the room. Bruce stayed by Tony's side, keeping an eye on the potion. Steve plopped down on the floor beside the cauldron, wringing his hands together. Bucky, Sam, Natasha, and Clint were huddled in a corner playing a quiet game of cards. Peggy stood watch by the door.

Tony studied the students scattered around the classroom, wondering to himself how in the world half of them had even ended up here. With him, of all people. He sighed. This had better work. For all he knew, he might even get some friends out of it.

After an hour had passed, Tony was pleased to find that the potion was now a light, electric blue.

"It's ready!" he proclaimed. In an instant, everyone dropped what they were doing and rushed to the center of the room.

"It looks like slime," Bucky remarked, peering over the edge of the pot.

"That's what most potions look like," Sam reminded him.

"In case this goes... er, poorly," Tony broke in, "I want the rest of you out in the hall." He waved his hand to dismiss their indignant cries. "I'll be staying to make sure everything goes ok."

"Good luck, mate," Sam murmured, gifting Steve with a hearty slap across the shoulder.

"See you on the other side," Peggy added, giving his arm a reassuring squeeze. Tony watched as Clint, Natasha, and Bruce dipped their heads in a solemn farewell. Bucky was the last to leave, wrapping Steve's small frame in a quick embrace. When they had pulled away from one another, Bucky turned towards Tony.

"Don't mess this up," he grunted.

With that, Bucky brushed past him and followed the rest of the group into the corridor outside.

"You ready?" Tony asked, unable to keep the nervous jitter out of his voice. Steve nodded, seeming not to notice. The boy was pacing back and forth as Tony grabbed a vial and began ladling the potion.

He let out a slow, deep breath, then turned around. With shaking hands, Tony handed the vial to Steve. The boy took it, staring at the blue liquid with a look that straddled the line between terror and anticipation.

"You ready?" Steve asked. Tony hesitated.

"Yeah," he said, scratching the back of his neck. "I'm ready."

By instinct alone, Tony stepped away from Steve until his back rested against the warm cauldron. Steve gave the potion one last glare, then downed it in a single gulp. Tony waited, his heart racing.

Nothing.

Steve looked down at his thin frame, disappointment crossing his face.

Then he threw his head back, a terrible scream escaping. Tony watched in horror as Steve collapsed to the ground, writhing across the tile as his body convulsed and shuddered. Tony could see the changes in the boy's face, and abdomen, and limbs. His arms and legs were stretching and growing. His chest was expanding. His face widened to fit the broad shoulders he now wore. There were horrible screams and cries coming from the boy, but Tony could do nothing but wait.

It was over as soon as it had began. Steve's eyes rolled back into his head as he slumped to the floor one final time, unconscious. Tony surveyed the boy on the ground below with awe, yet there was no time to process.

"That's it, I'm going in."

The door crashed open and Peggy entered the room. Bucky was right behind her, trailed by Sam and the rest. They all stopped once they crossed the threshold, gaping at Steve's body on the floor.

"Holy mother of -"

"It worked!"

"Is he alive?"

Tony reached down to check for a pulse, gesturing for all of them to stay by the door.

"He's got a pulse," he yelled. "I think he's stabilized."

"We should get him to the hospital wing," Peggy cried.

"Tony!" Bruce shouted all of a sudden. "The cauldron!"

Tony turned towards the cauldron, just a few scarce feet away. The metal was starting to glow, and he could see smoke rising from inside. Panic ensued.

Bolting towards the cauldron, Tony grabbed his wand.

"Augamenti!" A stream of water shot from the end of his wand, but it wasn't enough. Tony approached the cauldron, preparing another spell on the tip of his tongue.

Before the words could leave his mouth, there was a thunderous bang, and then a blinding flash of light. A hot, searing pain tore through Tony's chest and he found himself being thrown backwards. He hit something, probably a desk, and fell to the ground.

Everything was blurry. The pain was unbearable. He could hear voices all around him, but they were muffled and nonsensical. Spots swam before his eyes. The noises, the voices, the pain... everything started to fade.

Then it all turned to black.