Dumbledore summons the entire school to the Great Hall. He announces that the students will sleep there while teachers search the castle, and he conjures sleeping bags. Percy shouts that the lights will go out in ten minutes, so Harry, Ron, and Hermione drag sleeping bags into a corner. Hermione and Ron note that they're lucky Sirius Black tried to break in tonight, since nobody was in the tower for him to hurt. They hear a Ravenclaw suggesting that Black Apparated into the castle, which annoys Hermione: according to Hogwarts: A History, a person can't Apparate in, and the dementors see through disguises.
Keep in mind that Sirius Black isn't actually after Harry and doesn't want to murder him. With this information, it becomes clear that Black specifically chose to sneak in tonight when there was nobody there, as people aren't his goal. Hermione's derision about the Ravenclaw student shows that she believes that what she reads in books is entirely true and doesn't question information when it comes from that kind of a source.
At about three in the morning, Dumbledore finds Percy near Harry, Ron, and Hermione. They pretend to sleep so they can listen. Dumbledore whispers to Percy that he's found a temporary guardian for the portrait hole, and Snape arrives to say that Sirius Black is nowhere to be found. Snape asks if Dumbledore has any theories as to how Black got in, and reminds Dumbledore of a conversation they had in which Snape expressed "concerns." Dumbledore, in a tone of finality, says that nobody in the castle would've helped Black get in. Percy asks if the dementors wanted to help search the castle, but Dumbledore replies that dementors won't enter the castle while he's around.
Dumbledore's reply to Snape implies that Snape thinks someone is helping Black enter the castle, which shows that Snape is just as suspicious of people as Harry, Ron, and Hermione are of him. Remember though that in the Shrieking Shack, Lupin will admit that he did inadvertently help Black by keeping it secret that he's an Animagus. However, because the trio distrusts Snape, they won't take him seriously here even though, in a way, he's right to be suspicious of Lupin.
Over the next few days, the rumors about Sirius Black get wilder and wilder. Sir Cadogan replaces the Fat Lady, which annoys all the Gryffindors—he changes passwords all the time and challenges people to duels. Much to Harry's annoyance, teachers start to walk with him in the corridors and Percy trails him as well. Then, McGonagall summons Harry to tell him first that Black is after him, and second that she doesn't think Harry should be practicing Quidditch in the evenings. Harry is distraught, but McGonagall agrees to let him practice if Madam Hooch supervises.
When she insists that Harry be under adult supervision at all times, McGonagall shows that she takes her responsibility as a teacher and as Harry's Head of House very seriously. In the same vein, it's also worth noting that while McGonagall's lessons are hard, it's implied that, like Lupin, she has reasonable lesson plans. This suggests that being a good teacher and a caring teacher go hand in hand.
A few days before the first Quidditch match, Oliver tells the team that they won't play Slytherin as planned since Malfoy is still "injured." They'll play Hufflepuff instead. He's worried because their new captain and Seeker, Cedric Diggory, is very good. At the mention of Diggory, Angelina, Alicia, and Katie giggle that he's handsome. Oliver is distraught, but Fred assures him they'll take Hufflepuff seriously. Oliver tries to talk strategy to Harry every chance he gets over the next few days. This makes Harry late to Defense Against the Dark Arts.
Because Malfoy is able to create the reality that works for him (by pretending his arm still hurts), he's able to change how events happen in a very meaningful way. This again reminds the reader of the power that some people have to turn things into reality, even when that "reality" isn't actually real at all.
Snape greets Harry instead of Lupin, docks points from Gryffindor for Harry's lateness, and explains that Lupin is ill. He tells the class that Lupin's lesson plans are disorganized and tells Hermione to be quiet when she lists the creatures they've already studied. Both Dean and Parvati try to help Hermione and defend Lupin, but Snape menacingly tells the class to turn to the chapter on werewolves. Nobody except Hermione raises their hand when he asks how to distinguish between a werewolf and an actual wolf and when Hermione tries to answer, Snape docks more points because she's "an insufferable know-it-all." Ron asks why Snape even asked the question, which earns him detention, and they spend the rest of class silently making notes on werewolves. At the end, he asks for a long essay on werewolves, due Monday.
When the class chooses to stand up to Snape, it shows that Lupin's teaching and the way he treats them has given them confidence to stand up for themselves and advocate for their education. Further, when Snape calls Hermione names, it shows again that he cares little for his students and cares only for his own agenda. While Snape's choice to teach the class about werewolves seems a strange one, Harry and Ron learn later that he's actually trying to tip them off about Lupin—he's trying to give the class information that would effectively discredit their teacher.
The next morning, Harry wakes early to a draft on his neck—Peeves is blowing in his ear. Peeves zooms away, cackling, and Harry decides to get up. He stops Crookshanks from entering the dormitory and listens to the noisy storm outside. As Harry thinks of Cedric Diggory, who is much heavier than Harry is, he begins to feel apprehensive—Diggory is less likely to be blown off course. Finally, Harry heads down to the locker rooms with the team.
Because Harry is able to look objectively at his strengths compared to Cedric Diggory's, he's then able to come up with a plan to make the game easier for himself. In this way, having more information means that Harry can more clearly see what's going on and deal with what's happening.
Harry worries he won't be able to see, as the rain covers his glasses. The wind is so loud that he can't even hear Madam Hooch tell the players to mount their brooms and start flying. Harry is frozen within minutes. He lands when Oliver waves at him; Oliver tells him to get the Snitch soon. Hermione appears as though on cue and puts a charm on Harry's glasses so they repel water. The lightning and thunder get worse and as lightning lights up the stands, Harry sees an enormous dog. He nearly falls off his broom but refocuses on the game when Oliver yells and points to Diggory, who's chasing the Snitch.
Hermione's charm on Harry's glasses brings the novel's exploration of perspective into the real world: she literally makes it so that Harry can see. However, Harry's still not seeing the truth—his fear at seeing the dog suggests he thinks it's the Grim still, when in reality, the dog is Sirius Black, a friend. In this case, the extra information actually has negative consequences.
As Harry chases Diggory, he notices the noise of the storm diminishing and suddenly feels a familiar sense of cold. He looks down and sees a hundred dementors below him. Harry hears a woman screaming and thinks he needs to help her as he starts to fall. He comes to in the hospital wing listening to his teammates talk about things that make no sense. Harry opens his eyes and asks what happened.
Harry's desire to help the woman he hears screaming shows that he cares deeply for others and wants to do what he can to protect people from pain and suffering. When he faints after this thought, it indicates that his altruism can also have negative consequences.
Fred and Angelina explain that Harry fell off his broom, and George says that Diggory caught the Snitch right after. Diggory tried to demand a rematch when he realized Harry had fallen, but even Oliver agrees that Hufflepuff won fairly. Fred and George talk through Gryffindor's chances of winning the Cup and then Madam Pomfrey kicks the team out. Hermione and Ron stay and tell Harry that Dumbledore was furious with the dementors. Harry is barely listening; he's thinking about the screaming in his head. Harry asks about his broom, and Hermione and Ron hesitantly tell him that it blew into the Whomping Willow. Hermione dumps broom pieces onto Harry's bed.
Diggory's attempts to organize a rematch offer the novel's first suggestion that true justice can really only come from individuals, not large groups or the government. While everyone agrees that this is unnecessary in this situation, this introduces Harry to the idea that individuals have the power to change things and make things right.