It is another summer away from Hogwarts and Harry Potter is back living with his awful Uncle Vernon and Aunt Petunia, and his cousin Dudley. Unexpectedly, Harry and Dudley come under attack by terrible demonic beings called “Dementors,” and when Harry utilizes magic to defend himself and Dudley, he is placed under intense scrutiny by the Ministry of Magic and almost expelled from Hogwarts for using magic in front of muggles in their world when he is still under age.

Harry is taken to 12 Grimmauld Place in London, which is both the headquarters of the Order of the Phoenix, as well as Sirius Black’s ancestral home. The Order is a group of wizards that is led by Albus Dumbledore, and its mission is to fight and stand against Lord Voldemort and his evil followers. Because the head of the Ministry of Magic, Cornelius Fudge, is corrupt and refuses to believe that Voldemort has returned, the Order has to do everything in secret and is not sanctioned by the Ministry.

Harry is ordered to stand trial because of his use of magic outside the school. He is vindicated with the help of Dumbledore, and returns to Hogwarts for another year with his best friends, Hermione and Ron. At Hogwarts, however, things have changed. Dolores Umbridge, who is a member of the Ministry of Magic and who voted against Harry at his trial, is slated to be the new teacher for Defense Against the Dark Arts. There is a lot of internal strife at Hogwarts, and the newspapers keep on defaming Harry and his actions by printing false stories about him. What is worse, Harry is now in his fifth year at Hogwarts and will have to sit for the O.W.Ls, which stands for “Ordinary Wizarding Level” examinations, causing him much stress.

Dolores Umbridge does not teach any spells to her students, and they learn nothing new in how to defend themselves. Meanwhile she is appointed “High Inquisitor” of Hogwarts by Cornelius Fudge, ostensibly to evaluate the professors and students. In protest, Harry, Ron and Hermione decide to form their own group to defend against the dark arts. They style themselves as “Dumbledore’s Army,” and manage to get twenty-five other students to join them as well.

Harry wants to get the word out to Sirius Black about what’s going on at Hogwarts, but he is unable to do so since Umbridge is examining all outgoing and incoming Owl mail a well as other methods used for contact. Umbridge then bans Harry from the Quidditch team. Fred and George, Ron’s brothers, leave Hogwarts in protest and they open a joke shop in London.

Harry is having recurring nightmares where he is walking in a corridor in the Ministry of Magic. He sees many spherical glass orbs, but always wakes up before he can ascertain what they are or what they are trying to show him. Another night he dreams that he is a large snake and attacks Ron’s father.

Professor McGonagall knows that something is amiss, and she brings him to see Dumbledore, who raises the alert to save Ron’s father from Voldemort’s evil. Dumbledore then turns to Snape to teach Harry Occlumency, which will help him to defend his mind against Lord Voldemort’s attacks. Unfortunately, Harry is not very successful at Occlumency as he is unable to empty his mind of every though and cannot close it off from peripheral influences. His link to Voldemort is demonstrated every single time his scar burns when Voldemort is feeling an extreme emotion.

Dumbledore’s Army hold regular meetings and their skills are improving rapidly. However, Umbridge catches them one day, and Dumbledore is forced to resign when he takes responsibility for all the group has done. Umbridge is then appointed headmaster. At this point, the student O.W.L examinations begin.

Harry has a new vision of his Godfather Sirius tortured and imprisoned by Voldemort in the Department of Mysteries in the Ministry of Magic. He does not heed Hermione’s warnings that this is a trap, as he is too worried about Sirius. After sneaking into Umbridge’s office, Harry is then transported to 12, Grimmauld Place, but Kreacher, Black’s house elf, informs Harry that Sirius is being held at the Ministry. When Harry goes back to Hogwarts, he finds out that his friends and he are caught in Umbridge’s office. Hermione and Harry trick Umbridge into following them into the woods by saying that they have hidden a great weapon of Dumbledore’s. Once there, centaurs spirit Umbridge away. Harry and his friends now get onto winged horses (thestrals) and fly off to the Ministry.

At the Ministry, Harry is unable to locate Sirius, and he is forced to admit that Hermione might have been right. He sees a glass orb with his and Voldemort’s name on it, and when he grabs it, he is surrounded by Death Eaters who want him to hand it over. The team from Hogwarts has limited success in fighting off the Death Eaters but is mostly outmatched. Fortunately, members of the Order of the Phoenix arrive to aid them. In this fight, the orb is smashed, and Sirius Black is killed by Bellatrix Lestrange, his own cousin.

Dumbledore arrives and subdues the Death Eaters, but Voldemort and Lestrange manage to escape as Cornelius Fudge appears. When he sends Harry back to Hogwarts, Dumbledore tells Harry that the spherical orb was essentially a prophecy, and that Voldemort is unaware of a power that Harry possesses, which is that of love. The prophecy states that there are two possible outcomes: Voldemort destroys Harry, or Harry destroys Voldemort. Dumbledore then reveals to Harry why he has to return every summer to his Uncle and Aunt. His mother had saved him with love, and only blood is able to seal that pact. His mother’s sister, Aunt Petunia, took Harry in and kept the pact alive, and as long as Harry stayed with them, he would be safe. At the end of the text, Harry returns to the Dursley’s for the summer.