The night after Black's break-in, all the students sleep in the Great Hall, while the professors, led by Dumbledore, search the castle for Black. They find nothing. Sir Cadogan replaces the Fat Lady as the Gryffindor entrance portrait, and he spends the days making up complicated passwords and challenging students to duels.
The weather takes a turn for the worse, and Gryffindor House approaches its first game against Hufflepuff, who has always been an unformidable opponent; still, Oliver Wood, the Gryffindor captain, warns his team to take Hufflepuff seriously. The day of the match, Harry enters his Defense Against the Dark Arts class to find Snape substituting, snarling at students, and scrapping Lupin's lesson plans in order to teach the class about werewolves.
Harry and the Gryffindor team take the field, and through the rain and wind it is impossible to see anything. Harry and Cedric Diggory, the Hufflepuff seeker, circle around each other looking for the Snitch, but with no avail. All of a sudden, Harry catches sight of a large black dog silhouetted against the bleachers; the next moment he spies the Snitch, and finally, as he pursues it on his trusty Nimbus 2000 broomstick, he looks down to see hundreds of Dementors milling around alongside the field. Harry feels intensely cold, hears the scream of his mother as Voldemort murdered her, and falls out of the sky. He wakes in the hospital wing to find that Cedric caught the Snitch, the Hufflepuffs won the game. Harry's Nimbus had landed in the Whomping Willow, an angry thrashing tree in the Forbidden Forest, and had been ruined.
After the fall from his broomstick, Harry is miserable, not only about the loss of his Nimbus or the game, but also about the strange effect of the Dementors on him, and his continued sightings of the Grimm right before disaster strikes. Lupin returns to teach around this time, excuses them from Snape's overload of werewolf homework, and after class explains to Harry that the Dementors affect him because his past is more horrible than other people's pasts, and the Dementors are creatures designed to drown humans in misery and despair. Finally, Lupin promises to teach Harry an anti-Dementor spell after Christmas, so Harry's spirits lift.
Around this time, a second Hogsmeade visit rolls around, and Harry is approached by Fred and George Weasley, who surreptitiously introduce him to a map that locates all Hogwarts' rooms, passages, and even people. It is called The Marauder's Map. It is invisible on parchment until the viewer taps it and says "I solemnly swear that I am up to no good." It is signed by four names: Padfoot, Prongs, Moony and Wormtail. Harry uses the map to navigate his way under the school and into the basement of Honeydukes, a sweetshop in Hogsmeade, where he encounters Ron and Hermione. Ron is delighted to see Harry, but Hermione is worried that Black might have access to Harry, or to the tunnels through which Harry came. Together they load their arms with wizard treats and then walk to the Three Broomsticks Pub, where they drink hot butterbeer served by Madam Rosmerta. All of a sudden, Cornelius Fudge enters the pub with several Hogwarts teachers, including Hagrid, McGonagall, and Professor Flitwick. Harry hides underneath a table and overhears as they discuss Black's role not only in killing thirteen people, a dozen innocent Muggles and a wizard named Peter Pettigrew, but also in betraying Harry's parents. Fudge explains to Rosmerta and the teachers that Black had been Harry's parents' best friend Secret-Keeper, under a charm that is impervious if the Secret-Keeper is faithful. Black had given away the secret of Harry's parents whereabouts to Voldemort, who in turn found and killed them. Thus, Black is the cause of the death of Lily and James Potter.
Snape's experience teaching Defense Against the Dark Arts is symbolic, not only because he has always wanted the job, but also because he demands that the class learn to recognize and kill werewolves. This is certainly aimed to inform them about Lupin's true form, and to ultimately have the school fire Lupin on account of student fear. Snape is as nasty as ever, and here he is truly in his element; later in the book we are granted a better idea of why exactly Snape is so distasteful to Harry, his friends, and to Lupin, but now we just see pure, unadulterated conniving bitterness. Chapter nine marks the lowest point of Harry's time at Hogwarts this year. He loses his first Quidditch game, loses his beloved broomstick, and caves in when faced with Dementors. Quidditch has been the only outlet where Harry feels positively able to succeed, and for his breaking of his winning streak due to the already vulnerable situation with the Dementors, is the worst possible thing that could happen.