Snape has found Harry's cloak at the base of the Whomping Willow. He accuses Lupin of helping Black. He charms some cords which tie themselves around Lupin. He commands all of them to leave towards Hogwarts. Harry opposes him. At the same moment, Harry, Ron and Hermione disarm him with a Expelliarmus, Snape is knocked out from the blow. Black unties Lupin. Black had recognised Scabbers as Pettigrew when he saw the photo of the Weasleys in Egypt in the Daily Prophet. The rat is missing one toe and they only found one finger of Pettigrew's after the blast. Crookshanks has been helping Black to find and capture Scabbers. According to Lupin and Black, it was Pettigrew who betrayed his parents. Twelve years ago, at the last moment, Black decided that Pettigrew should be their Secret-Keeper since nobody would expect they would use a weak wizard like him. This decision cost Harry's parents their lives. Lupin and Black transform Scabbers back into Pettigrew by magic. Pettigrew has in fact been hiding all these years, waiting to see if Voldemort would return.
Black tells he has been able to escape from Azkaban only because he knew he was innocent. This wasn't a happy thought so the Dementors couldn't suck it out of him, but it kept him sane; helped him keep his powers. He could still transform into a dog and managed to escape that way, as Dementor's have much more trouble's sensing animal emotions.
Lupin and Black decide to kill Pettigrew. Harry stops them. He wants Pettigrew to be handed over to the Dementors, and his life is spared by Harry's decision. Lupin straps Ron's leg with the charm ¤ Ferula. Snape is still out cold and they use a Mobilicorpus charm to take him with them. Appearing as though invisible strings were tied to his wrist, neck and knees, he is pulled into a standing position like a puppet.