Harry starts off his third year at Hogwarts rather eventfully when he inadvertently blows up his Aunt Marge, goes on the run, and is then personally absolved by the Minister of Magic himself. He then learns that mass murderer, Sirius Black, is intent on killing him, and Mr. Weasley makes him promise a strange thing, that no matter what he hears he won’t go looking for Black. Confused, Harry agrees.
As he heads back to school armed with this knowledge, he learns that dementors, evil, soul-sucking creatures, have been stationed around Hogwarts to protect the students from Black. The dementors affect Harry much more than the other students because of his terrible past, and he quickly resolves to learn to defend himself when he begins collapsing every time they come near. He finds a willing teacher in Professor Lupin, the new Defense Against the Dark Arts professor, who begins teaching Harry the Patronus Charm, which is the only spell that will work against the foul creatures.
As the school year progresses, the third year students are buried under piles of homework. Hermione is already stressed out due to her enormous course load, and none of them can figure out how she’s getting to all her classes. It doesn’t help that Hermione’s cat, Crookshanks, repeatedly tries to eat Ron’s rat, Scabbers, causing the two friends to argue ceaselessly about the animals. They stop speaking entirely when it’s discovered that Crookshanks seems to have eaten Scabbers and Ron is devastated.