Harry and his crew make a desperate attempt to contact Sirius via the fireplace in Umbridge's office, but the Black family's house-elf, Kreacher, deliberately tricks Harry into believing Sirius has vanished; he was in fact tending to Buckbeak, the hippogriff. Umbridge and her minions--mainly Slytherin students--capture Harry's gang. Thinking fast, Hermione makes up a story about who they were trying to contact, and says that they were protecting a weapon, and that she and Harry would lead Umbridge to it. Umbridge asks Snape to give her some Veritaserum, but he claims he has none left; she is not aware that he is also a member of the Order of the Phoenix. Umbridge tells Harry that it was she who ordered the Dementors on him during the summer.

逃往伦敦

Hermione entices Umbridge to march her and Harry into the woods, knowing that the haughty centaurs, furious that one of their brethren now works for humans, are disposed to hate adult wizards. The centaurs take Umbridge away, and are about to do the same to Harry and Hermione, when Hagrid's half-brother - a "small" giant named Grawp, whom Hagrid had brought back with him from his quest over the summer - appears and distracts the centaurs. Ron, Ginny, Luna and Neville arrive, and they resolve to go to the Ministry to rescue Sirius, riding on thestrals, horse-like creatures visible only to people who have seen death.

Upon arriving at the Department of Mysteries, and after a number of false turns, they arrive at the location in Harry's dream, to find not Voldemort and Sirius but a group of Death Eaters. The Death Eaters include, among others, Draco's father Lucius Malfoy; MacNair the executioner in the employ of the Ministry; Azkaban escapee and former Department of Mysteries employee Rookwood; and Azkaban escapee Bellatrix Lestrange, who is Sirius's cousin and the torturer of Neville Longbottom's parents. The vision of Voldemort and Sirius at the Department of Mysteries was a trap. When Voldemort realised that Harry's dreams gave him access to Voldemort's mind, he planted the vision of Sirius being tortured to lure Harry to the Ministry and use him to retrieve a glass sphere, the record of a prophecy, from the Department of Mysteries. The prophecy, made before Harry's birth, is apparently about Voldemort and Harry. Only the subjects of a prophecy can handle it, and as Voldemort does not want to risk discovery, he lures Harry into retrieving it for him. Having previously heard the first part of the prophecy relating to Harry's ability to conquer the Dark Lord - on which basis Voldemort attempted unsuccessfully to murder Harry as a baby, precipitating his own downfall - Voldemort is now determined to hear the full prophecy, hoping to learn how to destroy Harry.

魔法部里的混战

A great skirmish begins between the students and the Death Eaters. Most of the students are seriously injured, but as they near defeat, five of the adult wizards from the Order of the Phoenix appear to help them, including Sirius. During the ensuing battle, the glass sphere which holds the prophecy is shattered. A ghostly image pronounces the prophecy but no one can hear it amidst the fighting. Also, tragically, Sirius is struck by a curse from the wand of his cousin, the Death Eater Bellatrix Lestrange. He falls through a veil inside an arch in the Death Chamber of the Department of Mysteries, and vanishes. Dumbledore shows up and ropes off most of the Death Eaters, but Bellatrix escapes.

Harry blindly chases after Bellatrix, intent on avenging Sirius, and soon catches up with her in the main atrium of the Ministry of Magic, where they fight. Then Lord Voldemort himself appears inside the atrium. He and Dumbledore duel, and after a dramatic fight and a brief episode where Voldemort actually manages to possess Harry, tempting Dumbledore to kill him in Harry's body, Voldemort eventually retreats by disapparating, taking Bellatrix with him. Alerted Ministry of Magic employees arrive in time to see He Who Must Not Be Named for themselves. Among them is Cornelius Fudge, who finally accepts Voldemort's return and believes what Dumbledore and Harry have been saying. In turn, The Daily Prophet reverses its hostile attitude towards the pair, restoring their reputations and praising them for warning the wizarding community of Voldemort's return.

The Prophecy

As the story draws to a close, Dumbledore explains much to Harry. He did not wish to be close to him during the year, as he could sense Voldemort's growing power over the boy--indeed, on more than one occasion, Harry was filled with a desire to strike down Dumbledore. He regretted not helping Harry learn Occlumency, and also reveals the prophecy. It turns out that at her initial Hogwarts interview, held sixteen years ago at the Hog's Head Inn at Hogsmeade, Sybill Trelawney prophesied that:

The one with the power to vanquish the Dark Lord approaches...born to those who have thrice defied him, born as the seventh month dies...and the Dark Lord will mark him as his equal, but he will have power the Dark Lord knows not...and either must die at the hand of the other for neither can live while the other survives...the one with the power to vanquish the Dark Lord will be born as the seventh month dies...

This was interpreted to mean that Voldemort would kill either Harry or Neville Longbottom, who were both born at the end of July, or vice-versa. After Voldemort, hearing of the prophecy, attacked Harry as a child, the latter part of the prophecy was realised; Harry would be his foe and kill or be killed by him, not Neville.

The existence of this prophecy brings clarity to Dumbledore remarking that Trelawney's prediction in Harry Potter and the Prisoner of Azkaban has been her second real one. The first prediction was the Harry/Voldemort prophecy.

Dumbledore also tells why Harry must stay with the Dursleys each summer. After Harry's mother died to save him, the charm of protection against Voldemort that her act cast on Harry is sustained as long as Harry has a home among those who share Lily Potter's blood. Aunt Petunia, Lily's sister, therefore acts as a protector for Harry, and it was Dumbledore who sent Petunia the Howler at the beginning of the story to remind her of this.

最后，哈里又回到了德思礼家，but not without Vernon and Petunia getting a stern talking-to by a number of wizards in the Order (including a menacing Alastor Moody). Changes are brewing in the wizarding world. Indeed, the last chapter of the book is entitled "The Second War Begins."