Harry Potter and the Prisoner of Azkaban is a partial mystery novel with a series of secrets and inadvertent lies, which may be the most dangerous kind. It seems like nearly everyone has a secret in this book: Harry tries to keep his reaction to the Dementors under wraps; Hermione doesn't tell even her BFFs about her Time-Turner; and all the adults try to keep the "truth" about Sirius Black a secret from Harry. It's notable that the secret of Sirius Black is actually a lie; the real secret there is only known by Peter and Sirius for much of the novel. Secrets and lies, both intentional and accidental, start overlapping in this novel and raise a lot of complicated questions: is it ever OK to lie to your friends? Do secrets protect people, or can they do more harm than good? The themes of secrets and lies are summed up nicely with the idea of the Secret Keeper, which played a crucial role in the fate of Harry's parents.