1. Dementor Attack: The book opens with Harry being attacked by Dementors while on his way to his Muggle cousin Dudley's house. He uses a Patronus charm to fight off the Dementors, but his illegal use of magic outside Hogwarts leads to him being expelled.
2. Order of the Phoenix: Sirius Black, Harry's godfather, informs him that the Ministry of Magic is trying to suppress reports about Voldemort's return. To combat this, Dumbledore forms the Order of the Phoenix, a secret organization dedicated to fighting Voldemort and his followers.
3. Ministry Propaganda: The Ministry discredits Harry's account of Voldemort's return and labels him a liar and attention seeker. The Daily Prophet, a wizarding newspaper, runs negative articles about Harry, further tarnishing his reputation.
4. Defense Against the Dark Arts: Dolores Umbridge, a senior official from the Ministry, is appointed as the new Defense Against the Dark Arts professor at Hogwarts. She refuses to teach students practical defensive spells, claiming that theoretical knowledge is sufficient.
5. Student Rebellion: Frustrated by Umbridge's methods and the Ministry's unwillingness to accept the truth about Voldemort, Harry, Hermione Granger, and Ron Weasley form an underground student organization called Dumbledore's Army. They meet secretly to learn proper defense against the Dark Arts.
6. A prophecy revealed: Harry learns from a prophecy hidden in the Department of Mysteries that either he or Voldemort must kill the other to survive. Their fates are inextricably linked.
7. Battle at the Department of Mysteries: Voldemort lures Harry and his friends to the Department of Mysteries, seeking the prophecy he believes will guarantee his immortality. A tense battle ensues between Dumbledore's Army and Death Eaters, Voldemort's followers. Sirius Black dies during the conflict, devastating Harry.
8. Truth Revealed: Harry is exonerated after the Ministry of Magic witnesses Voldemort's return. Dumbledore confirms that Voldemort attempted to possess Harry as a baby but failed.
9. Aftermath and cliffhanger: The book ends with the Wizarding World on high alert, knowing that a significant battle against Voldemort and his forces is inevitable.
"Harry Potter and the Order of the Phoenix" explores themes of friendship, courage, and the importance of fighting for what's right, amidst a climactic battle that changes the course of the series.