During another summer with his Aunt Petunia and Uncle Vernon, Harry Potter is expelled from Hogwarts after using magic to save himself and Dudley Dursley from Dementors. Harry is whisked off by a group of wizards including Mad-Eye Moody, Remus Lupin and several new faces including Nymphadora Tonks and Kingsley Shacklebolt to Number 12, Grimmauld Place, the home of his godfather Sirius Black. The building also serves as the headquarters of the Order of the Phoenix, a secret organisation founded by Albus Dumbledore which informs Harry that the Ministry of Magic is refusing to believe Lord Voldemort’s return. At the Order’s headquarters, Sirius mentions that Voldemort is after a special object he did not have during his previous attack.
Harry’s expulsion is reversed at a hearing of the Ministry of Magic and he returns to Hogwarts for his fifth year, where Minister of Magic Cornelius Fudge has appointed a new Defence Against the Dark Arts professor: Dolores Umbridge. Umbridge immediately clashes with Harry and punishes him for his “lies” about Voldemort by forcing him to write a message with a magic quill, scarring his hand. Despite the concern of his best friends Hermione Granger and Ron Weasley, Harry refuses to report Umbridge’s punishment to Dumbledore, who has mysteriously been ignoring Harry since the summer. As Umbridge’s control over the school increases, Harry, Hermione and Ron form a secret group to train students in defensive spells, calling themselves “Dumbledore’s Army”. Umbridge tasks the Slytherin students with exposing the group, forming the “Inquisitorial Squad”. Meanwhile, Harry and Cho Chang develop romantic feelings for each other.
Harry has a vision involving an attack upon Arthur Weasley, from the point of view of Arthur’s attacker. Concerned that Voldemort will exploit this connection to Harry, Dumbledore instructs Severus Snape to give Harry Occlumency lessons to defend his mind from Voldemort’s influence. These end after Harry discovers through Snape’s memories why Snape hated Harry’s father James (who often ridiculed him). The connection causes Harry to further isolate himself from his friends. Meanwhile, Sirius’ deranged Death Eater cousin Bellatrix Lestrange escapes from Azkaban along with nine other Death Eaters. At Hogwarts, Umbridge and her “Inquisitorial Squad” expose Dumbledore’s Army by forcing Cho to drink Veritaserum. Dumbledore escapes as Fudge orders his arrest. Umbridge becomes the new Headmistress.
Harry’s refusal to forgive Cho causes their relationship to fall apart. After seeing another vision where Sirius is tortured by Voldemort, Harry, Ron and Hermione rush to Umbridge’s fireplace to alert the Order via the Floo Network but are caught by her. When Umbridge threatens to use the Cruciatus Curse on Harry, Hermione tricks her into entering the Forbidden Forest in search of Dumbledore’s “secret weapon”. Hermione and Harry lead Umbridge to the hiding place of Rubeus Hagrid’s giant half-brother Grawp, only to be confronted by centaurs who kidnap Umbridge after she attacks and insults them.
Harry, Hermione, Ron, Ginny Weasley, Luna Lovegood, and Neville Longbottom fly to the Ministry of Magic on Thestrals in an attempt to save Sirius. The six enter the Department of Mysteries and uncover a bottled prophecy, the object Voldemort was after. However, they are ambushed by Death Eaters led by Lucius Malfoy and Bellatrix. Lucius reveals that Harry only saw a dream of Sirius being tortured, which served as a ruse to lure Harry into the Death Eaters’ grasp. Harry refuses to give Lucius the prophecy, causing a fight between Dumbledore’s Army and the Death Eaters.
The Death Eaters capture everyone except Harry, threatening to kill them unless he surrenders the prophecy. As Harry obliges, the Order arrive and attack the Death Eaters. This causes Lucius to drop the prophecy, destroying it. Just as Sirius overpowers Lucius, he is killed by Bellatrix. Voldemort appears, but Dumbledore arrives through the Floo Network moments before he can kill Harry. A duel between Voldemort and Dumbledore ensues, which destroys much of the Atrium while Bellatrix escapes. Unable to defeat Dumbledore, Voldemort possesses Harry to tempt Dumbledore into sacrificing him but the love Harry feels for his friends and Sirius makes it impossible for Voldemort to remain in his body.
Ministry officials arrive right before Voldemort disapparates; Fudge is forced to admit Voldemort’s return and is forced out of his position as Minister in disgrace. Umbridge is removed from Hogwarts, and Dumbledore returns as headmaster. Dumbledore explains that he distanced himself from Harry throughout the year, hoping it would lessen the risk of Voldemort using their connection. Harry comes to terms with the prophecy: “Neither can live while the other survives.”