Harry and his unpleasant cousin Dudley ar
e suddenly attacked by Dementors in Little Whinging (suburbia). Harry fends them off with a Patronus Charm. Harry receives an owl letter stating he has been expelled from Hogwarts for performing magic outside school. Several more letters arrive in quick succession: Arthur Weasley and Sirius Black warn Harry not to leave the house, while another overturns his expulsion and orders him to appear at a hearing at the Ministry o Magic.
Several Order of the Phoenix members escort Harry to their secret headquarters at the Black Family home, 12 Grimmauld
Place in London. The Weasleys, Hermione, and Harry’s godfather Sirius Black are there. There Harry learns that Voldemort plans to build up an army and is attempting to retrieve a "weapon". Harry is cleared of the charges against him, because he acted in self-defence. The Ministry, however, still refuses to admit that Voldemort has returned.
The students return to Hogwarts, where Dolores Umbridge, Senior Undersecretary to the Minister, has been appointed by the Ministry as the new Defence Against the Dark Arts teacher. She refuses to teach students any practical defense methods, only using textbooks to teach Ministry-approved theory. It is soon clear she is only there to spy on and take control of the school. She is soon appointed High Inquisitor, intimidating staff and students. She also harbours an intense dislike for Half-breeds, Centaurs, werewolves, and similar creatures. She considers Hagrid (a half-breed giant) and Sybill Trelawney incompetent, and fires Trelawney.
Harry has been having strange dreams of running down a hallway and attempting to open a door in the Ministry of Magic's Department of Mysteries. He also dreams he is a snake attacking Ron's father, Arthur Weasley. The next morning, Arthur is discovered covered with venomous snake bites. Harry fears he is being possessed and transported by Voldemort to do his bidding. Dumbledore makes Severus Snape give Harry lessons in the art of Occlumency to block his mind from intrusion.
Hermione blackmails journalist Rita Skeeter into writing a newspaper article that Harry witnessed Voldemort's return. Luna Lovegood's father publishes the article in his magazine The Quibbler. Furious, Umbridge bans the tabloid at the school, but the story spreads rapidly, gathering support for Harry.
Hermione convinces Harry to secretly teach students Defence Against the Dark Arts. They name their group the Defence Association (D.A.), although they begin calling themselves "Dumbledore's Army" to mock the Ministry of Magic, which fears that Dumbledore is trying to gain power for himself. Umbridge uncovers the secret meetings, and to protect the students, Dumbledore claims he organized the group. Confronted by two Ministry Aurors (Dawlish and Shacklebolt), Fudge, Percy Weasley, and Umbridge, Dumbledore is spectacularly whisked away on his phoenix, Fawkes. Umbridge is appointed Headmistress; the other professors do nothing to help her regain control over the school.
Harry receives a vision that Sirius is being tortured at the Department of Mysteries, unaware it is a ruse. He desperately attempts to contact Sirius via the fireplace in Umbridge's office but is caught. Umbridge reveals it was she who sent the Dementors to attack Harry during the summer. As she is about to use the Cruciatus Curse on him, Hermione tells her that Dumbledore has hidden a powerful weapon in the Forbidden Forest. Umbridge goes there, insults a Centaur, and gets captured by the other Centaurs.
Harry, Ron, and Hermione, along with Luna, Ginny, and Neville use the school's Thestrals to fly to the Ministry of Magic in London to save Sirius, unaware they are being lured into a trap. When they arrive at the Department of Mysteries, they are ambushed by Death Eaters. Voldemort seeks a Prophecy contained in a glass sphere and needs Harry to retrieve it for him.
The students heroically fight the Death Eaters, but they are outnumbered. Just as they are nearly defeated, Order members arrive. During the ensuing battle, the glass sphere is shattered. Sirius is attacked by his cousin, Bellatrix Lestrange, a Death Eater. He falls backwards through a mysterious veiled archway, never to return. Soon, Dumbledore arrives and most of the Death Eaters are captured. Lord Voldemort appears and duels with Dumbledore. Ministry of Magic employees arrive on the scene in time to see Voldemort before he disapparates, taking Bellatrix with him. Cornelius Fudge finally admits Voldemort has returned, and Harry's interview with Rita Skeeter is reprinted in the Daily Prophet.
Later in his office, Dumbledore apologizes to Harry for neglecting him over the past year. He reveals the lost prophecy's meaning, which states Harry must kill Voldemort or be murdered by him. Sybill Trelawney predicted, "Either must die at the hand of the other, for neither can live while the other survives." At the end of the year, Harry returns to the Dursleys for another summer, deeply grieving for his lost godfather.
