1. Harry's Return to Privet Drive and the Ministry's Denial: Harry returns to a summer of neglect and disbelief from the Ministry of Magic, who dismiss his warnings of Voldemort's return as "delusions" and "lies."
2. The Formation of Dumbledore's Army: Frustrated by the Ministry's inaction, Harry forms a secret group of students to learn defensive magic, despite the danger and illegal nature of the organization.
3. The Battle at the Department of Mysteries: Harry and his friends are drawn into a confrontation with Death Eaters in the Department of Mysteries, where they discover crucial information about Voldemort's plans. This battle results in the death of Sirius Black.
4. The Trial of Harry Potter: Harry is put on trial for using magic outside of school and falsely accused of attacking Ministry officials. He is acquitted, but the trial exposes the Ministry's corruption and bias.
5. The Rise of Umbridge and the Censorship of Magic: Dolores Umbridge, the new Defense Against the Dark Arts teacher, enforces strict control over the curriculum, suppressing any real magical learning and creating an atmosphere of fear.
6. The Ministry's Incompetence and Corruption: The Ministry, under the control of Cornelius Fudge, actively undermines Dumbledore and dismisses any evidence of Voldemort's return, leading to dangerous consequences.
7. The Prophecy and the Battle for the Ministry: The revelation of a prophecy that connects Harry and Voldemort sets the stage for a major battle at the Ministry, where Dumbledore and his Order face off against Voldemort and his Death Eaters.
8. The Capture of Sirius Black and His Death: Sirius Black, believing that Harry is in danger, rushes to the Ministry, only to be killed by Bellatrix Lestrange. This traumatic event highlights the growing threat and the personal losses Harry faces.
9. Dumbledore's Departure and Harry's Struggle with Grief: Dumbledore leaves Hogwarts to deal with a critical issue, leaving Harry emotionally drained and burdened by the events of the year.
10. The Return of Voldemort and the Growing Darkness: The book ends with Voldemort's public return and the growing tension and fear throughout the wizarding world, setting the stage for the next book in the series.
These events highlight the central themes of the book: denial, resistance, the fight against corruption, the power of belief, and the escalating conflict between good and evil.