Harry Potter and the Prisoner of Azkaban · Chapter 20
Buckbeak's Rescue and Sirius's Escape
Harry and Hermione use a Time-Turner to save Buckbeak from execution and help Sirius Black escape from the castle before the Dementors can administer the Kiss.
Outcome
Buckbeak and Sirius are saved, and Harry learns he cast the powerful Patronus that drove away the Dementors.
