Snape sneers and says that he found the invisibility cloak where Harry had dropped it, at the base of the Whomping Willow. He proceeds to tie up Lupin and point his wand at Black, threatening to kill him. Snape refuses to hear the full story because he is set on vengeance. Finally Harry, Ron and Hermione all disarm him at once, knocking him out. Black then explains that he recognized Scabbers as Peter Pettigrew in a photo in the newspaper Cornelius Fudge gave him during a visit, and he set out to Hogwarts after him. He explains Scabbers' missing toe by saying that Peter must have cut off his own finger before blowing up the street and disappearing into the gutter. Black adds that Hermione's cat Crookshanks had collaborated with him in pursuit of Peter.
Even with these explanations, Harry is frightened and skeptical, and so, to clarify things once and for all, Lupin reverses Scabbes' form into that of a short, balding man—Peter Pettigrew. Peter is squirmy and nervous and pleads with each of the people in the room to believe his innocence and let him go, and only after many more facts are clarified does Harry believe that Sirius Black is innocent, and that Peter Pettigrew did in fact support Voldemort and turn Harry's parents over to him. Peter finally admits, after much evasion, that he is and was the servant of Lord Voldemort. Lupin and Black prepare to kill Peter, when Harry stops them by saying that his dad probably wouldn't have wanted his two best friends to become murderers. They all tie up Peter and secure him between Lupin and Ron, and they bring with them the unconscious Snape as together they troop back to Hogwarts.
As the group returns through the tunnels, Black informs Harry of his role as his godfather. He invites Harry to live with him, once his name is cleared. Harry is thrilled at the idea of leaving the Dursleys. Once they emerge from the ground into the forest, the clouds above them shift, revealing a full moon. Lupin turns into a werewolf, wrangling out of the chain binding him to Peter, and Black transforms into a dog and fights the werewolf away, into the woods. During the commotion, Peter grabs Lupin's wand, casts some stupefying spell on Ron, and turns himself back into a rat. He escapes, and Black then tears back into the woods after Scabbers.
Harry and Hermione examine the unconscious Ron, declaring him alive; they are interrupted by the yowl of a dog, and they turn to see Black in human form, cowering at the approach of over a hundred Dementors. Harry and Hermione run to help, but none of Harry's patroni are strong enough to hold back the throng. Hermione drops next to him, depressed and exhausted, and Harry feels himself weakening. Suddenly one of the Dementors lowers its hood, revealing gray skin and empty eye sockets and a shapeless sucking hole of a mouth, and the Dementor grasps Harry's head and prepares to administer the soul-sucking kiss to him. Harry feels himself drowning in cold and misery, when all of a sudden a bright shape begins to drive the Dementors away. The air is warm again as they retreat, and Harry looks across the lake to see a large shimmering animal galloping away to a man who looks oddly familiar, like Harry's father. Exhaustion then overtakes Harry, and he faints.
Chapter nineteen is virtually all explanation. Certain mysteries, such as how Black escaped from Azkaban or why he only ever pursued Ron, suddenly fall into place. Once again, nothing is exactly how it seemed; the usual suspects are innocent, those presumed dead suddenly appear alive. In Rowling's writing, culprits are never easy to spot, although she does give us certain hints along the way. In this book, this principle is especially important, as so much of the plot involves misguided perception and its effects on the surrounding community. The only true culprit in this book was the one who was living all the while in Ron's pocket.
Animagi are key to this chapter; none of the four makers of the Marauder's Map could have lived the full richness of their experiences without the ability to become animals at will. This age-old idea in mythology and literature opens a range of new possibilities. This book places an emphasis on liminal figures who exist on a boundary between two identities. These figures include Harry and Black, who touch both the Magical and the Muggle world, and Lupin, who has at once a tame, genteel nature coupled with a potential bout of insanity at every full moon.
These chapters present a vivid and telling glimpse into the past of Hogwarts. Harry watches Black transform to keep Lupin in check, as he must have done many years before, and he sees the presence of someone who looks faintly like his father at the other side of the lake, moments after a patronus saves him from the Dementors. Peter Pettigrew is just as he must have always been, squealing and hysterical, nervous and full of flattery. Black is sharp-edged but loyal, we learn, and he is deeply brave in his fight against the werewolf in order to save Harry, Ron, Hermione, and-sort of-Snape. Hogwarts contains so many mysteries and the traces of so many students who have lived there, known the grounds, and left tracks for Harry and his friends to decipher, that the chance to see them return to life, even as middle-aged versions of the once boisterous lads, is a rare form of magic.