"It was as if someone had lit a fire In my head, and the dementors couldn't destroy it.... It wasn't a happy feeling... it was an obsession... but it gave me strength, it cleared my mind. So, one night when they opened my door to bring food, I slipped past them as a dog.... It's so much harder for them to sense animal emotions that they were confused.... I was thin, very thin... thin enough to slip through the bars.... I swam as a dog back to the mainland.... I journeyed north and slipped into the Hogwarts grounds as a dog. I've been living in the forest ever since, except when I came to watch the Quidditch, of course. You fly as well as your father did, Harry...." 

He looked at Harry, who did not look away.

 "Believe me," croaked Black. "Believe me, Harry. I never betrayed James and Lily. I would have died before I betrayed them."

Harry saw the pained expression lessen on Amelia's face at these words.And at long last, Harry believed him. Throat too tight to speak, he nodded. 

"No!"

 Pettigrew had fallen to his knees as though Harry's nod had been his own death sentence. He shuffled forward on his knees, groveling, his hands clasped in front of him as though praying. 

"Sirius -- it's me... it's Peter... your friend... you wouldn't --" 

Black kicked out and Pettigrew recoiled. 

"There's enough filth on my robes without you touching them," said Black. 

"Remus!" Pettigrew squeaked, turning to Lupin instead, writhing imploringly in front of him. "You don't believe this wouldn't Sirius have told you they'd changed the plan?"

 "Not if he thought I was the spy, Peter," said Lupin. "I assume that's why you didn't tell me, Sirius?" he said casually over Pettigrews head.