In the process of returning to Hogwarts, Harry learns of an escaped lunatic from the wizard prison Azkaban, Sirius Black. He goes through the normal pre-school trials and tribulations, but on the Hogwarts Express, he is accosted by a dementor while in his compartment with Ron, Hermione, and a sleeping man. The man turns out to be the new Defense Against the Dark Arts professor, Remus Lupin, and he sends the dementor away. School is slightly different for the trio this year, as Hogwarts is being guarded by the dementors word gets out about the problem on the train, and the Slytherins, led by Draco, mock him mercilessly until the Quidditch match scene, where Harry is attacked by dementors again. Professor Lupin helps Harry learn to deal with them, teaching him the Patronus charm, which serves him well in the final scenes. In the end, Harry finds something he didn't know he had ever lost.