For twelve long years, the terrible fortress of Azkaban held prisoner an infamous prisoner named Sirius Black. Convicted of killing thirteen people with a single curse, he was said to be the heir apparent of the Dark Lord, Voldemort. Now he's escaped, leaving only two clues as to where he might be headed: Harry Potter's defeat over You-Knows-Who was also Black's downfall. And the guards of Azkaban heard Black muttering in his sleep, "He's at Hogwarts... he's at Hogwarts." Harry Potter is not safe, not even within the walls of his wizarding school, surrounded by his friends. Because, on top of all that, there may be a traitor among them.