Harry spends a boring summer between his fourth and fifth years of wizarding school at his terrible Aunt and Uncle's house. He is angry and fed up with the fact that none of his friends are telling him any news about the war against Voldemort. Harry and his cousin, Dudley, are suddenly attacked one night by a group of dementors, evil magical creatures with the ability to suck out souls. Although he is not allowed to use magic outside of school, Harry casts a spell which drives the dementors away. He is then sent a message saying he must attend a hearing at the Ministry of Magic. If he is found guilty with the charge of using magic outside of school, then he will be expelled.
Harry is then brought by a group of wizards to London, where Harry's godfather Sirius's house is. The house, called number twelve, Grimmauld Place, is also the base of the rebel group "The Order of the Pheonix." Led by Albus Dumbledore, the group of adult wizards fight against the evil wizard Voldemort and his supporters. Their actions are unbeknownst to the government, which is led by Cornelius Fudge, who continues to insist that Harry is delusional and Voldemort has not returned.
Harry goes to his hearing. Although Fudge is determined to have Harry expelled, Harry has an eyewitness to prove that he casted the spell out of self defense. Harry's neighbor, Arabella Figg, is a squib, which means she is a non-magical person with magical parents. She knows Dumbledore, and has been watching out for Harry ever since he was a baby. Thanks to her testimony, and Dumbledore's, Harry is cleared of all charges.
Harry and his friends Ron and Hermione return for their fifth year at Hogwarts School of Witchcraft and Wizardry. There they learn that the new Defense Against the Dark Arts teacher is Dolores Umbridge, a woman who is an employee of Fudge. Harry finds that many of his classmates believe him to be mentally unstable. This is because Fudge has been pressuring the wizarding newspaper the Daily Prophet to publish untrue stories that discredit Harry so no one will believe his story that Voldemort has truly returned.
Harry and his classmates quickly form a dislike for Professor Umbridge. She refuses to teach her students defensive magic. Not only this, but she also tells them Harry is a complete liar. This causes Harry to have many fights with her in class, which leads to her giving him many detentions. During these detentions, Harry must write lines with a pen that uses his blood as its ink. Fudge soon appoints Professor Umbridge as the High Inquisitor of Hogwarts, which gives her much more control. She soon begins to reform the school, and passes many decrees that restrict students' freedoms. She also gives Harry a life-long ban from the wizarding sport Quidditch. At this time, Hagrid returns from a failed mission to get the giants on the side of the Order of the Pheonix. After inspecting all the teachers, it seems clear Umbridge intends to get rid of either Hargrid, the divination teacher Professor Trelawney, or both.
In retaliation to Umbridge, Harry and his friends form a secret defense group called Dumbledore's Army (the name is a joke, and Dumbledore has no relation to their group). Led by Harry, twenty-five students meet as often as they can and learn defensive spells.
During this time, Harry begins to have strange dreams about walking down a corridor with a door at the end of it. He always wakes up before he realizes the meaning of his dream. He also begins to feel pain in his scar whenever Voldemort experiences an extreme emotion. Near Christmastime, Harry has a vision where he watches from a snake's eyes as it attacks Ron's father. He alerts Professor McGonagall and she takes him to Dumbledore. Dumbledore speaks to the portraits on his wall, and they are able to make sure that Mr. Weasley is found and saved. Harry is becoming increasingly concerned about his connection with Voldemort. So is Dumbledore, and he insists Harry takes Occulumency lessons with Professor Snape. This is the practice of closing off ones mind from outside invasions. Harry is very poor at this, and does not seem to be able to improve.
Umbridge finally fires Professor Trelawney, but Dumbledore allows her to stay at Hogwarts. A little later, the student Marietta Edgecomb tips off Umbridge about Dumbledore's Army, and they are caught in the middle of a meeting. Harry is taken up to Dumbledore's office where Fudge is waiting. Dumbledore, however, takes full blame for the secret society and is forced to step down from his position as Headmaster. Umbridge takes over his position, and Ron's brothers Fred and George leave the school in protest.
During one of his O.W.L. exams (Hogwarts fifth year exams), Harry has a vision of Sirius being held and tortured by Voldemort in the Department of Mysteries. He is extremely distressed and he and his friends break into Umbridge's office to use her unpoliced fireplace to transport to number twelve Grimmauld Place. However, all Harry finds there is Kreacher, who tells him Sirius has been taken to the Ministry of Magic. Harry returns from the fireplace only to realize his friends have been captured by Umbridge and her squad. Umbridge asks Snape to get her Veritaserum, a potion which will force Harry to tell the truth about who he was speaking with. Snape, however, says he has none. Knowing Snape is an Order member, Harry attempts to send Snape a message that Sirius is in trouble. He is not sure Snape understood or will do anything about it, however.
So, Harry and Hermione quickly devise a plan where they lure Umbridge into the Forbidden Forest, where she is carried away by Centaurs. He, Hermione, Ron, Ginny, Neville, and Luna all mount thestrals and travel to the Ministry of Magic. Once there, they get to the Department of Mysteries and realize that Voldemort had been tricking Harry - Sirius is not there.
There are many glass orbs in the Department of Mysteries, which are full of prophecies. One has Harry's name on it. He picks it up and many Death Eaters (Voldemort supporters) suddenly appear around the group, demanding Harry hand over the prophecy. Using their defense skills, the students are able to hold them off until the Order of the Pheonix arrives. During this fight Sirius is killed, much to Harry's devastation. Harry drops the prophecy and it shatters.
Dumbledore and Voldemort appear at around the same time. They have a duel where Voldemort attempts to possess Harry in order to get Dumbledore to kill him, but he does not. Fudge and other ministry representatives arrive and Voldemort escapes. Fudge is forced to admit that Voldemort has returned, and reinstates Dumbledore as headmaster.
Once back at Hogwarts, Dumbledore explains many things to Harry. He tells him Snape understood Harry's warning and alerted members of the Order of the Pheonix. Dumbledore also explains that the prophecy had said that a boy would be born in July with the ability to defeat Voldemort. It also said that neither could live while the other survived. Voldemort had heard part of this prophecy and attempted to kill Harry to prevent it from becoming true. However, by doing this, he gave Harry the power to defeat him and sealed his fate. Dumbledore says Harry must go back to his Aunt and Uncle's house this summer, because when Harry's mother died, she protected him with her love. As long as Harry is in the home of Aunt Petunia, his mother's sister, he is safe.
Although he is still mourning Sirius, Harry knows this will do him no good with the fight against the Death Eaters quickly approaching. So, says goodbye to his friends and returns to his Aunt and Uncle's house.