Prisoner of Azkaban. The third year of Harry's time at Hogwarts reveals some of the darker aspects of magic.

When Harry's parents were killed by Voldemort, they were in hiding, as they expected to be attacked. In order to make their hiding place secret they made one of their close friends their Secret Keeper. Noone can learn a secret kept by a magical Secret Keeper, unless the Keeper tells them personally. In order to find them Voldemort must have been told where they were. After the attack when Voldemort was as good as killed, Harry's father's closest friend Sirius Black was cornered by the magical police in a crowded street, and thirteen bystanders died before they took him in. Black has escaped after eleven years in Azkaban, the wizard prison, and for months he's been heard to mutter in his sleep 'he's at Hogwarts'. Because of the serious risk of Black heading for Hogwarts, perhaps to try to kill Harry, the Minister for Magic Cornelius Fudge has allowed the Dementors, hideous creatures who guard Azkaban, to enter Hogwarts grounds. The Dementors suck emotions from their victims, feeding on happiness and forcing bad memories to the surface. Harry is affected particularly badly due to the pain in his distant past.

The year passes, and Harry learns to fend off the Dementors with the Patronus Charm, taught to him by Professor Remus Lupin, the new Defence Against the Dark Arts teacher. Lupin turns out to be a friend of Harry's father, and of Sirius Black. Black does indeed show up at Hogwarts, evading all security measures to enter the school. Hiding out at the Shrieking Shack in Hogsmeade, the village below Hogwarts castle, Black turns out to be an unexpected ally. He and Lupin were James Potter's closest friends, along with a wizard named Peter Pettigrew, who Sirius supposedly killed before he was arrested. Rather than Sirius being Secret Keeper for the Potters, Pettigrew had been, he was the one who betrayed Harry's parents. He did not die when he killed the people Sirius was accused of murdering, but changed form in the confusion and has been hiding out as Scabbers, Ron's pet rat. All four friends had been able to change form, since Lupin became a werewolf and the others had become Animagi to keep him company at full moon. Pettigrew escapes, and returns to Voldemort, fulfilling the first part of a prophecy uttered by Professor Trelawny, the Divination teacher, earlier that year.