Harry Potter and the Prisoner of Azkaban is the third of a planned series of seven books (see annotation of Harry Potter and the Sorcerer's Stone for an introductory summary). Harry, forced to suffer another summer with the Dursleys, has just turned thirteen. When Uncle Vernon's sister arrives and proceeds to abuse Harry, he rebels, runs away with his heavy school trunk and is picked up by the Knight Bus, a wizard transportation vehicle. Meanwhile, the nine-member Weasley family, usually short of money, have won a wizard lottery and are using the money to visit the eldest son, Bill, in Egypt.

Sirius Black, whose motorcycle was featured in the first chapter of the first book, has escaped Azkaban and the prison guards known as Dementors. Sirius was imprisoned just after the death of Harry's parents when he was caught at the scene of another horrendous crime. Special precautions for Harry's safety are arranged by Dumbledore and the Ministry of Magic, led by Cornelius Fudge. When Harry meets a Dementor on the train to Hogwarts, he blacks out as he feels a rush of coldness, a complete lack of happiness or future, and relives his worst memories. Remus Lupin, the mysterious, gentle and periodically ill Defense of the Dark Arts Professor, provides the antidote: chocolate.

Thus begins Harry's third year at Hogwarts. Hermione signs up for an especially busy, seemingly impossible, schedule of classes. Ron's old pet rat, Scabbers, takes a turn for the worse, despite Ron's attention and care. The invisibility cloak again proves useful, as does a magical map. Hagrid, cleared of the cloud that had been hanging over him since his school days, is promoted to teacher: Care of Magical Creatures. However, an injury to Draco Malfoy by Buckbeak the hippogriff (a flying bird-horse) during the first class leads to another investigation.

Bizarre characters, such as the doom-predicting Divination teacher, Sibyll Trelawney, exciting Quidditch matches with a new broomstick for Harry, more run-ins with Snape, and a peek at Hogsmeade, an all magic village, round out the story. Ron, Hermione, and Harry's dangerous adventure leads to the exposure of Sirius Black, the truth of his connection to Harry's parents, and new discoveries for Harry about his father. Our heroes also discover who is the servant to Voldemort, the Dark Lord.