J. Rowling - Harry Potter and the Prisoner of Azkaban

Then Hermione spoke, in a trembling, would be calm sort of voice, as though trying to will Professor Lupin to talk sensibly.

“But Professor Lupin… Scabbers can’t be Pettigrew… it just can’t be true, you know it can’t…”

“Why can’t it be true?” Lupin said calmly, as though they were in class, and Hermione had simply spotted a problem in an experiment with Grindylows.

“Because… because people would know if Peter Pettigrew had been an Animagus. We did Animagi in class with Professor McGonagall. And I looked them up when I did my homework—the Ministry of Magic keeps tabs on witches and wizards who can become animals; there’s a register showing what animal they become, and their markings and things… and I went and looked Professor McGonagall up on the register, and there have been only seven Animagi this century, and Pettigrew’s name wasn’t on the list.”

Harry had barely had time to marvel inwardly at the effort Hermione put into her homework, when Lupin started to laugh.

“Light again, Hermione!” he said. “But the Ministry never knew that here used to be three unregistered Animagi running around Hogwarts.”

“I you’re going to tell them the story, get a move on, Remus,” said Black, who was still watching Scabbers’s every desperate move. “I’ve waited twelve years, I’m not going to wait much longer.”

“All right… but you’ll need to help me, Sirius,” said Lupin, “I only know how it began…”

Lupin broke off. There had been a loud creak behind him. The bedroom door had opened of its own accord. All five of them stared at it. Then Lupin strode toward it and looked out into the landing.

“No one there…”

“This place is haunted!” said Ron.

“It’s not,” said Lupin, still looking at the door in a puzzled way. “The Shrieking Shack was never haunted… The screams and howls the villagers used to hear were made by me.”

He pushed his graying hair out of his eyes, thought for a moment, then said, “That’s where all of this starts—with my becoming a werewolf. None of this could have happened if I hadn’t been bitten… and if I hadn’t been so foolhardy…”

He looked sober and tired. Ron started to interrupt, but Hermione said, “Shh!” She was watching Lupin very intently.

“I as a very small boy when I received the bite. My parents tried everything, but in those days there was no cure. The potion that Professor Snape has been making for me is a very recent discovery. It makes me safe, you see. As long as I take it in the week, preceding the full moon, I keep my mind when I transform… I’m able to curl up in my office, a harmless wolf, and wait for the moon to wane again.

“Before the Wolfsbane Potion was discovered, however, I became a fully fledged monster once a month. It seemed impossible that I would be able to come to Hogwarts. Other parents weren’t likely to want their children exposed to me.

“But then Dumbledore became Headmaster, and he was sympathetic. He said that as long as we took certain precautions, there was no reason I shouldn’t come to school…” Lupin sighed, and looked directly at Harry. “I told you, months ago, that the Whomping Willow was planted the year I came to Hogwarts. The truth is that it was planted because I came to Hogwarts. This house—” Lupin looked miserably around the room, “—the tunnel that leads to it—they were built for my use. Once a month, I was smuggled out of the castle, into this place, to transform. The tree was placed at the tunnel mouth to stop anyone coming across me while I was dangerous.”

Harry couldn’t see where this story was going, but he was listening raptly all the same. The only sound apart from Lupin’s voice was Scabbers’s frightened squeaking.

“My transformations in those days were—were terrible. It is very painful to turn into a werewolf. I was separated from humans to bite, so I bit and scratched myself instead. The villagers heard the noise and the screaming and thought they were hearing particularly violent spirits. Dumbledore encouraged the rumor… Even now, when the house has been silent for years, the villagers don’t dare approach it…”

“But apart from my transformations, I was happier than I had ever been in my life. For the first time ever, I had friends, three great friends. Sirius Black… Peter Pettigrew… and, of course, your father, Harry—James Potter.”

“Now, my three friends could hardly fail to notice that I disappeared once a month. I made up all sorts of stories. I told them my mother was ill, and that I had to go home to see her… I was terrified they would desert me the moment they found out what I was. But of course, they, like you, Hermione, worked out the truth…”

“And they didn’t desert me at all. Instead, they did something for me that would make my transformations not only bearable, but the best times of my life. They became Animagi.”

“My dad too?” said Harry, astounded.

“Yes, indeed,” said Lupin. “It took them the best part of three years to work out how to do it. Your father and Sirius here were the cleverest students in the school, and lucky they were, because the Animagus transformation can go horribly wrong—one reason the Ministry keeps a close watch on those attempting to do it. Peter needed all the help he could get from James and Sirius. Finally, in our fifth year, they managed it. They could each turn into a different animal at will.”

“But how did that help you?” said Hermione, sounding puzzled.

“They couldn’t keep me company as humans, so they kept me company as animals,” said Lupin. “A werewolf is only a danger to people. They sneaked out of the castle every month under James’s Invisibility Cloak. They transformed… Peter, as the smallest, could slip beneath the Willow’s attacking branches and touch the knot that freezes it. They would then slip down the tunnel and join me. Under their influence, I became less dangerous. My body was still wolfish, but my mind seemed to become less so while I was with them.”

“Hurry up, Remus,” snarled Black, who was still watching Scabbers with a horrible sort of hunger on his face.

“I’m getting there, Sirius, I’m getting there… well, highly exciting possibilities were open to us now that we could all transform. Soon we were leaving the Shrieking Shack and roaming the school grounds and the village by night. Sirius and James transformed into such large animals, they were able to keep a werewolf in check. I doubt whether any Hogwarts students ever found out more about the Hogwarts grounds and Hogsmeade than we did… And that’s how we came to write the Marauder’s Map, and sign it with our nicknames. Sirius is Padfoot. Peter is Wormtail. James was Prongs.”

“What sort of animal—?” Harry began, but Hermione cut him off.

“That was still really dangerous! Running around in the dark with a werewolf! What if you’d given the others the slip, and bitten somebody?”

“A thought that still haunts me,” said Lupin heavily. “And there were near misses, many of them. We laughed about them afterwards. We were young, thoughtless—carried away with our own cleverness.

“I sometimes felt guilty about betraying Dumbledore’s trust, of course… he had admitted me to Hogwarts when no other headmaster would have done so, and he had no idea I was breaking the rules he had set down for my own and others’ safety. He never knew I had led three fellow students into becoming Animagi illegally. But I always managed to forget my guilty feelings every time we sat down to plan our next month’s adventure. And I haven’t changed…”

Lupin’s face had hardened, and there was self disgust in his voice. “All this year, I have been battling with myself, wondering whether I should tell Dumbledore that Sirius was an Animagus. But I didn’t do it. Why? Because I was too cowardly. It would have meant admitting that I’d betrayed his trust while I was at school, admitting that I’d led others along with me… and Dumbledore’s trust has meant everything to me. He let me into Hogwarts as a boy, and he gave me a job when I have been shunned all my adult life, unable to find paid work because of what I am. And so I convinced myself that Sirius was getting into the school using dark arts he learned from Voldemort, that being an Animagus had nothing to do with it… so, in a way, Snape’s been right about me all along.”