1. The Chamber of Secrets is Opened: This event sets the entire plot in motion. Students are being petrified, and Harry discovers he is somehow connected to the opening of the Chamber. This leads to the investigation into the Chamber and its history, and Harry's eventual confrontation with the basilisk.
2. The Identity of the Heir of Slytherin is Revealed: The mystery of who opened the Chamber is a major driving force of the story. The reveal that Lucius Malfoy planted the diary on Ginny Weasley to manipulate her into opening the Chamber is a pivotal moment, revealing the depth of the conflict and the true nature of the threat. This also reveals the manipulative nature of pure-blood supremacists and the dangers of prejudice.
3. Harry Defeats the Basilisk: The climax of the story, this event sees Harry confront the basilisk, ultimately saving the petrified students and Hogwarts itself. His use of Fawkes's phoenix tears to heal the victims and his destruction of the basilisk using the Sword of Gryffindor showcase his bravery, resourcefulness, and ultimately, his destiny as a wizard.