I do not own Harry Potter. Charlotte Potter is my OC.

Explanations

"How can Scabbers be Peter Pettigrew?" snapped Ron. "Didn't you kill him?"

Sirius smiled wickedly. "I certainly tried. But it turns out Peter had a degree of cunningness that he had never shown before. He accused me of being the murderer, in front of all those witnesses, and executed the explosion himself."

"I think a lot of pieces are missing from the story," said Charlie. "I'm…still a bit confused."

"So am I," said Remus with a nod. "Sirius, if you please."

"Fine," said Sirius, his eyes narrowing. "Do not let that traitor go, Ron."

"I thought every Animagi had to be registered with the Ministry of Magic. There's a process, so that they'll be able to keep tabs on every Animagi form that exists," said Charlie, recalling McGonagall's class on the subject. "I helped Hermione go through the list. There's not a lot of them for this century, but Peter Pettigrew wasn't on there. Neither is Sirius Black."

"And neither is James Potter," said Remus patiently.

Charlie's eyes widened and Harry stood a little straighter. "Dad was an Animagi?" she asked softly.

"He was a magnificent stag," said Sirius proudly. "It's why we called him Prongs."

"So the three of you became Animagi illegally?" said Harry in disbelief.

The door creaked loudly and they all jumped. Remus slowly approached the door, his wand raised, but he didn't see anything on the landing. "We're fine."

"Isn't this place supposed to be haunted?" asked Ron nervously.

"Actually, the screams and howls the villagers heard from this shack were mine." Remus shut the bedroom door once more and turned to face them. "That's where the story starts, really. I was a small child when I was bitten by a werewolf, and back then there was no cure. The Wolfsbane Potion is a very recent discovery. As long as I drink it the week before a full moon, I keep my mind when I transform and can wait it out.

"Before the potion was discovered, however, I became a horrible beast once a month. It seemed impossible that I would ever get to go to Hogwarts. No one wanted their children to be around me. But then Dumbledore became Headmaster, and he said he didn't see why I couldn't attend so long as they took the necessary precautions."

"The Whomping Willow," Charlie said in realization. "You said the tree was planted the year you came to Hogwarts. It was planted because you were coming to Hogwarts, wasn't it?"

Remus nodded sadly. "The tunnels were constructed for my use. Once a month I would be brought down here to transform and the tree prevented anyone from coming across me while I was in werewolf form.

"To transform into a werewolf is very painful. As I was separated from humans, I bit and scratched myself. My howls were heard by the villagers, who believed them to be evil spirits and Dumbledore encouraged the rumour. But even with the transformations, I was the happiest I had ever been in my life. I had three wonderful friends—Sirius Black, Peter Pettigrew and of course, James Potter.

"They became Animagi so they could keep you company," said Charlie. "They wanted to be with you."

"The relationship I had with my friends is much like the one you have with Hermione, Ron and Harry," said Remus wistfully. "We spent so much time together it was inevitable that they would grow suspicious that I would sneak off once a month."

"Or disappears and reappears a few feet away," Harry muttered under his breath, and Charlie elbowed him in the side.

"I was terrified that they would leave me if they found out the truth. They discovered my secret, and all they wanted to do was find a way to make my transformations better. It took three years for them to accomplish it, but by fifth year they were able to come out every full moon with me."

"How would them becoming Animagi help you?" Ron asked.

"You see, werewolves are only a danger to people," explained Remus. "Not really to other animals."

"But I don't understand," said Harry, perplexed. "Uncle Remus, if you knew Sirius Black was an Animagus all this time, and you thought he was a traitor, why didn't you tell anyone when he first broke into Hogwarts?"

"I didn't want Dumbledore to find out," said Remus, his fingers clenching together. "I was a childish coward. I didn't want to betray the trust of the only man who was willing to give me a chance. I convinced myself that Sirius was getting into the castle using the Dark Arts he would have learned from You-Know-Who." He turned to Harry, his expression pained, and he whispered, "And I feared, if the Ministry found out I'd been keeping such secrets, they would find a reason to take you away from me. It seems, in a way, Snape has been right about me all along."

Sirius' eyes narrowed. "Snape?" he hissed.

"He's a teacher at Hogwarts, Sirius. He teaches the skills of Potions to Harry, Charlotte, Ron and their other friend Hermione," he informed him. "You see, the reason why Snape loathes me so much is because of a trick Sirius played on him that nearly got him killed. A trick that involved me."

"Did…did my dad save his life?" asked Charlie, remembering the life-debt Snape owed her father that he paid in her first year.

"He did," said Remus in surprise. "How did you know?"

"It doesn't matter. What happened?"

"Snape got curious as to where I went every month. He didn't like us and we didn't like him, but Sirius and James—"

"We despised him," growled Sirius.

Remus coughed awkwardly. "Yes. Snape loathed them in turn. But he didn't miss the opportunity when he spotted Madam Pomfrey escorting me to the Whomping Willow one night. Sirius thought it would be funny to tell him how to calm the tree. Snape, of course, took the chance. But he never made it past the house. James was horrified when he found out what Sirius had done. He went straight after Snape. In the end, he found out what I was, but Dumbledore forbade him from telling anyone."

Charlie was stricken. "That's…that's not a joke. That was a horrible thing to do."

"Indeed it was, Miss Potter," sneered a familiar, cold voice from behind Remus.

Everyone gasped as Severus Snape pulled Charlie's Invisibility Cloak from his body, wand pointing directly at Sirius and Remus.

Dang, thought Charlie in dismay. Probably should have been more careful with that.