The next day, Ministry wizards drive them to King's Cross station to catch the Hogwarts Express. Before Harry boards, Mr Weasley takes him to one side. He is about to break to Harry that Sirius Black might be after him, but Harry knows from the conversation he overheard the last night. Harry says he is not scared of Black, saying that he "can't be worse than Voldemort, can he?" Mr Weasley tries to get Harry to promise that he won't go looking for Black, but they are interrupted by the departure of the train.

Harry sits in a compartment with Ron, Hermione and a new teacher, Professor Lupin, who is fast asleep. Harry tells him about what he has heard from Mr Weasley. They also discuss Hogsmeade, with Harry explaining that he doesn't have permission to go on visits to the village. They get a brief visit from Malfoy, Crabbe and Goyle, but the presence of Lupin stops Malfoy from looking for trouble. The suddenly, the train grinds to a halt. It's too early for them to be at Hogwarts. Then the lights go out. Neville and Ginny blunder into the compartment and Lupin wakes up. Lupin produces a light and goes to the door, but something else opens it.

A mysterious cloaked creature appears in the doorway. It draws a breath that causes everything to go cold. Harry is overwhelmed and as he passes out, he hears a woman screaming in the distance. When he comes round, the creature has gone and the train is continuing on. Lupin explains it was a Dementor, one of the guards of Azkaban, which was searching the train for Sirius Black. After Harry passed out, Lupin cast a spell to drive the Dementor away. No-one else heard the screaming.

The train arrives. Harry and the others travel into the school in a fleet of carriages, passing more Dementors that are guarding the school. They get out, to find Malfoy is taunting Harry about how he fainted on the train. [=McGonagall=] takes Harry and Hermione aside and asks them to come to her office. She checks Harry is okay after the encounter with the Dementor. Harry is okay, apart from being embarrassed that he is the only person who fainted because of the Dementors. She then has a brief, private conversation with Hermione about her timetable. They arrive at the Great Hall after the Sorting ceremony has finished. Dumbledore makes a start of term announcement, warning that Dementors have been stationed at every entrance to the school. On a happier note, he welcomes two new teachers: Lupin for Defence Against the Dark Arts and Hagrid for Care of Magical Creatures.