It was three weeks after the Marauders' return to Hogwarts that a massive thunderstorm began to shake the castle, its dark clouds blotting out the full moon that James knew hung menacingly over them. He pressed himself against their dormitory's bay window with Peter and Sirius, all three of them waiting breathlessly for lightning to strike.

They ended up having to wait about thirty minutes before a streak of blinding light flashed before their eyes, striking somewhere within the depths of the Forbidden Forest. Peter gave a squeal of delight at the sight of it. "Finally!" James exclaimed, leaping to his feet with his heart suddenly pounding against his chest. "Come on—we've got to get outside before the storm ends."

He opened up his hidey-hole and pulled out the three phials of potion he'd stored there—each of them had turned an almost-sickening shade of blood-red, which he knew meant they were ready to drink.

The three Marauders climbed under James's Cloak and tore off through the castle as quickly as they could, shoving out of the Gryffindor common room and flying down multiple moving staircases before slipping through a side door behind a Rowena Ravenclaw portrait out onto the castle grounds. They'd already picked out a spot for their first transformation: a flat stretch of field near the Great Lake where they knew they couldn't be seen by anyone in the castle. Once there, they shrugged off the Invisibility Cloak and conjured up umbrellas to repel the heavy rain, pressing close against each other with their phials clenched in their hands.

"I'll go last," said James. A loud clap of thunder rumbled above them, making Peter jump. "So I can make sure you two get through it all right."

"I'll go first, then," Sirius said. He lowered his umbrella, his long hair soggy and dark against his skin, and raised his phial to his lips, swallowing the liquid inside with a single gulp. Peter grabbed at James, going tense; James, equally nervous, took his hand and squeezed it.

Sirius gave a little grimace, letting out his breath with a sudden exhale—then he began to transform, his bones crunching as they rearranged themselves and dragged him down towards the ground. Sirius's mouth and nose elongated into a large snout, and his ears were yanked up towards the top of his head. Dark fur sprouted all over his skin, thickening and lengthening until Sirius was entirely covered in it, and just like that he was a dog, big and black and long-tailed. He barked at Peter and James and began striding proudly in a wide circle around them.

James let out a whooping laugh. "Amazing, Sirius!" he called to the dog, who barked again in response. "Now it's your turn, Petey!"

Peter released James's hand, trembling. "I think I'm going to be sick," he whispered.

"Bollocks, Peter." James hunched down until he was at eye-level with his friend. "You're going to be fine. Sirius is, see?" Now the dog was spinning around after his own tail, snapping at it with long teeth.

"But I'm not like Sirius," Peter whispered. "Or you. I'm not a good wizard—you know that."

"Peter, we've worked on this for three years now," James said. Come on, Peter, please. "You can do it. I know you can do it. You can cast a Patronus, right? And you know you don't have to be a genius wizard to be an Animagus."

"I suppose," Peter muttered.

"Then go ahead and drink the bloody thing! It's for Remus, remember? We promised him we would help him." If everything went well tonight, James thought, this would be the last transformation Remus would have to endure on his own.

Swallowing, Peter gave a little nod and turned away from James. He closed his eyes, took a deep breath, and began drinking slowly from his phial.

Peter gave a little jolt as he finished, and the empty phial slipped from his fingers to fall into the grass at his feet. Instantly Peter began to shrink, growing smaller and smaller and thrashing as his body twisted into a new shapes until he was a sleek little rat, scurrying through the grass and letting out a series of loud, shrill squeaks.

"I knew you could do it, Petey," James said to him; he immediately felt ridiculous for speaking to a rat, but he was too thrilled by it all to care. And then it was his turn—James, much less nervous now that he'd seen his two friends transform, tossed away his umbrella and gulped down his potion as fast as he could. It tasted terrible, like drinking from a sewage pipe. The liquid burned as it seeped down his throat.

James felt his heart begin to race uncontrollably, a sudden pain coursing through him like fire. An image appeared in his head, as clear as if he were seeing it standing in front of him: a powerful red stag, with massive antlers and dark eyes that probed deep into James's. He stared back at it, remembering what McGonagall had said: he had to concentrate as hard as he could on the animal before he could merge with it. He took in every inch of its body, the glint off its short fur and the sharp points at the ends of its antlers, until the stag vanished and he could sense it somewhere inside of him, remolding his insides and transforming his body. James fell onto his hands and felt them curling inwards into hardened hooves, felt the antlers bursting from his forehead and the bones in his back shrinking and stretching and straightening. And finally the transformation was done, his bones and muscles resettled themselves, and James had become a giant deer.

His senses were different in his new body; his field of vision was wider, allowing him to see nearly all the way around him, and his nose was much more powerful—he could scent the rain in the air and the traces of countless animals in the field, including the tiny rat grooming his whiskers at his feet and the dog barking at him as he loped by. James felt a sudden, primal shock of fear at the sight of the dog's sharp teeth and lean muscles, but he pushed it aside and bounded over with his head down to ram his antlers against Sirius's flank. Yelping, Sirius twisted around and leapt onto James's back, pulling him to the ground; little paws pattered up his nose as rat-Peter climbed onto James's head and up into the maze of his antlers before leaping off him and burying himself in Sirius's shaggy black fur….

A moment later and the boys had all slipped back into their own skins again, their pajamas soaking wet and covered with grass stains as they tumbled over each other and laughed up into the stormy skies. "I can't believe we actually pulled that off," Sirius said breathlessly, rolling up to James's side.

James slammed a fist against Sirius's chest. "I can," he replied with a grin. "We're the Marauders, Sirius—we can do anything."

"Anything," Peter echoed from somewhere behind them. He sounded happier and more confident than James had ever heard him.

"Let's stay out here till dawn," James suggested. "We can surprise Remus when Pomfrey brings him out of the Whomping Willow. I can't wait to see the look on his face."