Harry Potter is spending another tedious summer with his dreadful Aunt Petunia and Uncle Vernon when a group of evil spirits called “dementors” stage an unexpected attack on Harry and his cousin Dudley. After using magic to defend himself, Harry is visited by a group of wizards and whisked off to number twelve, Grimmauld Place, London. Number twelve is the home of Harry’s godfather, Sirius Black, and the headquarters of the Order of the Phoenix. The Order is a group of wizards, led by Hogwarts headmaster Albus Dumbledore, dedicated to fighting evil Lord Voldemort and his followers. The Order is forced to operate in secrecy, outside of the jurisdiction of the Ministry of Magic, which is headed by the dense and corrupt Cornelius Fudge. Fudge refuses to believe that Lord Voldemort has returned.
Harry used magic to fight off the dementors, and since underage wizards are not permitted to use their wands outside of school, he must face a disciplinary hearing at the Ministry. With Dumbledore’s help, Harry is cleared and permitted to return to Hogwarts School of Witchcraft and Wizardry.
Reunited with his best friends, Ron and Hermione, Harry returns to Hogwarts and learns that Dolores Umbridge, an employee of Fudge, will be his new Defense Against the Dark Arts teacher. The Sorting Hat, which traditionally sorts all new students into one of four houses, cautions the students against becoming too internally divided. Meanwhile, the wizard newspaper, the Daily Prophet, continues printing untrue and unfair stories about Harry. Many of his classmates are whispering about him behind his back, but Harry ignores them and tries to concentrate on his studies, since all fifth-year students at Hogwarts are required to take O.W.L.s, or Ordinary Wizarding Level examinations.
Umbridge refuses to teach her students how to perform Defense spells, and before long, Fudge appoints her High Inquisitor of Hogwarts, giving her the authority to inspect all faculty members and evaluate their skills. In desperation, Harry, Hermione, and Ron form their own Defense Against the Dark Arts group, also known as the D.A., or Dumbledore’s Army. Twenty-five other students sign up, and they meet as often as possible to learn and practice Defense spells. Harry wishes desperately to contact his godfather Sirius to discuss the situation, but Umbridge is inspecting all Owl Mail and patrolling the fires that students can use to make contact with wizards residing outside of Hogwarts. Umbridge openly dislikes Harry, whom she considers a liar, and eventually bans him from the Gryffindor Quidditch team. Ron’s twin brothers, Fred and George, storm out of Hogwarts in protest, moving to London where they plan to open a joke shop using the money Harry won last year in the Triwizard Tournament.
Harry continues to have upsetting dreams about walking down a corridor at the Department of Mysteries, deep inside the Ministry of Magic. At the end of the corridor, Harry goes through several doors and enters a room full of dusty glass spheres. Harry always wakes up before he finds out what the dream means or what the spheres signify. One night, Harry has a vision where he inhabits the body of a large snake, and attacks Ron’s father. Harry wakes up horrified, and Professor McGonagall takes him to Dumbledore immediately. Dumbledore uses the portraits on the walls of his office to raise an alert, and Mr. Weasley is promptly rescued by two members of the Order. Dumbledore then demands that Harry take Occlumency lessons with Professor Snape, which should help Harry protect his mind against further invasions by Lord Voldemort.
Harry is unsuccessful at Occlumency because he has such difficulty clearing his mind of all thoughts, making it difficult for him to focus on closing his mind off to all outside influence. Meanwhile, his scar (from the attack in which Voldemort killed Harry’s parents) burns horribly every time Voldemort experiences a powerful emotion. The D.A. continues to meet regularly, and Harry’s peers show great improvement until they are caught by Umbridge. Dumbledore takes full responsibility for the group and resigns as Headmaster. Umbridge takes over his position. The students begin taking their O.W.L. exams, and Harry has another vision, this time about Sirius being held captive and tortured by Voldemort. Horrified, Harry becomes determined to save him. Hermione warns Harry that Voldemort may be deliberately trying to lure Harry to the Department of Mysteries, but Harry is too concerned about Sirius to take any chances.
Harry sneaks into Umbridge’s office, and, using her fireplace, transports himself to Twelve Grimmauld Place to look for Sirius. Kreacher, the Black house elf, tells Harry that Sirius is at the Ministry of Magic. Harry returns to Hogwarts to find that he and his friends have been caught in Umbridge’s office. Hermione and Harry convince Umbridge to follow them into the forest, where they claim to be hiding a weapon for Dumbledore. Once in the forest, Centaurs carry Umbridge away. Harry and his friends climb aboard flying horses called thestrals and speed off to the Ministry. Once they arrive, Harry cannot find Sirius and realizes that Hermione was right. Harry also sees that one of the glass spheres has his name on it, as well as Voldemort’s. Harry grabs the sphere, and Death Eaters surround to attack, demanding that Harry hand over the prophecy. Employing all of their Defense skills, Harry, Ron, Hermione, Ginny, Luna, and Neville have moderate success fighting the Death Eaters, but they are ultimately helped enormously by the arrival of several members of the Order. In the midst of the fight, Harry drops the glass sphere, and it shatters. Meanwhile, Sirius’ own cousin, Death Eater Bellatrix Lestrange, kills Sirius. Harry is devastated.
Dumbledore appears and corrals the Death Eaters. Voldemort and Lestrange escape, just as Fudge shows up. Dumbledore sends Harry back to school, where he explains how the sphere was a prophecy, which stated that Harry has a power that Voldemort does not know about. Dumbledore explains that this power is love. The prophecy goes on to claim that Harry will either destroy Voldemort or be destroyed by him. Dumbledore takes this opportunity to tell Harry why he must spend his summers with the Durselys in Little Whinging. Because Harry’s mother died to save him, he is blessed with her love, a blessing that can be sealed only by blood. Harry’s Aunt Petunia, his mother’s sister, makes that bond complete by taking Harry into her home. As long as he still calls Little Whinging home, Harry is safe. With this news, Harry returns to his Aunt and Uncle’s house for one more miserable summer.