Synopsis (may contain spoilers) Harry spends the beginning of his summer listening to the news for signs that Voldemort is back but hears nothing. He gets in trouble for hiding out under the living room window and gets angry with his aunt and uncle. He runs off to the park where and sees Dudley there. Harry and Dudley are attacked by dementors and Harry calls his Patronus to run them off. Their neighbor Mrs. Figg finds them and it is revealed that she is a squib.
Harry is expelled from Hogwarts because of his use of unauthorized magic. Aunt Petunia is startled by the mention of dementors and reveals her knowledge of some things magical. Dumbledore intervenes for Harry and he is no longer expelled. He must go to a hearing, though, at the Ministry of Magic.
Uncle Vernon and Aunt Petunia go out to a meeting with Dudley leaving Harry at home. While they are gone, several people arrive at Harry's house to escort him away from there. Remus Lupin and Alastor "Mad Eye" Moody are some of them and Harry meets Nymphadora Tonks, an auror.
Harry arrives at Number Twelve Grimmauld Place where he meets up with Ron and Hermione. He is very angry with them about not letting him know what's been going on. They say that Daumbledore made them promise not to. Harry also learns of the Order of the Phoenix, a group of wizards and witches dedicated to fighting Voldemort. Sirius Black is also there and Harry finds out that the Order's meeting place is actually Sirius' family house. Harry sees the Black family tree in one of the rooms and he learns that Sirius is not only related to the Weasleys and Tonks, but also to the Malfoys and the Lestranges.
The hearing date arrives and Arthur Weasley takes Harry to the Ministry of Magic. Mr Fudge still disbelieves him about the return of Voldemort. Dumbledore shows up to testify on Harry's behalf and then Mrs Figg testifies about the dementors. Harry is cleared of all charges even though Mr Fudge votes against him. On returning to Sirius' house Harry learns that Ron and Hermione have been chosen as prefects.
Harry and Ginny meet up with Neville on the Hogwart's Express while Ron and Hermione have to go to a meeting of the prefects. Harry meets Luna Lovegood and discovers that her father publishes a magazine called The Quibbler. When the train arrives at Hogwarts, Harry sees that the school carriages are pulled by creatures after all. Luna Lovegood can see them too.
Dolores Umbridge is introduced as the new Defense Against the Dark Arts teacher. Harry recognizes her as one of the people that votes against her at his trial. Harry discovers that not everyone believes him and Dumbledore about Voldemort. Harry, not liking the new DA teacher, and weary of being called a liar, gets assigned detention for a week by Professor Umbridge for an outburst in class.
Harry had seen on the first day that Hagrid was not at the ceremony. Professor Grubbly-Plank teaches Care of Magical Creatures because of this. Harry's serves his detentions, while Ron tries out for the Gryffindor Keeper position. Later Sirius talks with Harry through the fire in the Gryffindor common room. Professor Umbridge is made the High Inquisitor of Hogwarts. Harry loses his temper again about Professor Quirrell in Defense Against the Dark Arts. He has to serves another week's worth of detentions. Professor Umbridge watches the other professors in their classes to determine if they are fit to teach. Since Professor Umbridge is not teaching practical applications of DA, Ron and Hermione try to get Harry to give Defense Against the Dark Arts lessons. They have a meeting at The Hog's Head to talk with other students about the possibility of Harry giving them lessons. They all agree and sign the parchment that Hermione has. Professor Umbridge orders all Student Organizations, Societies, Teams, Groups and Clubs disbanded. Only groups approved by the High Inquisitor are allowed to exist. Sirius almost gets discovered in the Gryffindor common room fireplace talking to Harry. Dobby tells Harry about the Room of Requirement after he talks to the house elf about needing a place to meet secretly. The group meets there for the first time and decides to call themselves Dumbledore's Army.
Gryffindor plays Slytherin in Quidditch for the first time and Gryffindor wins. Afterward, Malfoy taunts the team and Harry, Fred and George attack him. They are given a lifetime ban from Quidditch by Professor Umbridge. Hagrid finally returns to school and he tells his story to Ron, Hermione and Harry. He and Madame Maxime had gone to find giants and were unsuccessful in gaining their alliance. Death Eaters, however, had a little more success. Hagrid's first class back is observed by Professor Umbridge.
Ginny Weasley gets Harry's job as Gryffindor's Seeker and Ron continues to do badly as the Keeper. Later Harry dreams about Arthur Weasley being bitten by a large snake in which Harry is the snake. He awakens and warns Professor McGonagall about it. Ron and Harry are taken to Professor Dumbledore's office where the pictures of past headmasters are able to find Arthur and he is rushed to St Mungo's Hospital. Dumbledore sends Harry and the Weasleys to Sirius's house with a portkey. They learn that he is going to pull through and they visit Arthur in the hospital the next day. Since it is Christmas Break, they all stay at the house and Hermione joins them for Christmas. Percy, who has been more and more loyal to Mr Fudge and his strong arm tactics, sends back the sweater that his mother made for him. They all visit Arthur again in St. Mungo's Hospital and see Gilderoy Lockhart there. They also see Neville with his grandmother visiting his parents. They find out what happened to his parents at the hands of the Death Eaters.
They all return to Hogwart's. Harry begins Occlumency lessons with Professor Snape in order to help him to be able to fight of Voldemort if he should try to possess him. The Daily Prophet reports that there is a mass breakout from Azkaban. Professor Umbridge bans all of the professors from giving students information no related to the subjects they are teaching. Harry and Cho go on a date which ends in a bad way. Hermione arranges for Harry to give an interview to Rita Skeeter (the former Daily Prophet columnist) and for his story to be published in Luna's father's paper, The Quibbler. After Harry's article is published in The Quibbler, the magazine is banned by professor Umbridge. Professor Trelawney, the Divination teacher, is fired and Firenze (a centaur whom Harry had met previously) is hired to replace her. Marietta Edgecombe tells Professor Umbridge of the DA's meeting but suffers from a jinx Hermione placed on the parchment they all signed at the Hog's Head. Dobby warns Harry that their meeting has been discovered. Harry is caught by Malfoy and denies the meeting to Mr Fudge and Professor Umbridge in Professor Dumbledore's office. Kingsley Shacklebolt modifies Marietta's memory and Dumbledore takes the blame. Before being arrested, Dumbledore stuns the Ministry agents and leaves Hogwarts. Professor Umbridge replaces Dumbledore as Headmistress of Hogwarts, although she cannot gain entrance to his offices. The students start to rebel with a little help from Fred and George. The teaching staff does little to help Professor Umbridge deal with the building crises. She tries to get Harry to tell her where Dumbledore is hiding with veratiserum. Harry is continuing his Occlumency lessons with Professor Snape but when he is called away Harry looks into the Pensieve. He sees one of Professor Snape's memories and finds out that his father may not have been as "perfect" as everyone makes him out to be. Harry wants to talk to Sirius and tells Ginny about it. She helps to arrange a way for him to break into Professor Umbridge's office to use the fire there (the only one not being monitored). Harry gets career advice from Professor McGonagall and says that he wants to become an auror. Harry gets to talk with Sirius about his father but Fred and George are discovered at their distraction and they leave Hogwarts to start their joke shop in Diagon Alley.
The Gryffindor team plays Ravenclaw for the house cup and win thanks to Ron finally getting the confidence he needs. Hagrid takes Harry and Hermione to meet his half-brother, Grawp. He makes them promise to look after him and teach him English if he is forced to leave the school. Harry and Hermione tell Ron about Grawp and confess that they didn't see him in the dame. The fifth year students take their OWLS. During Harry's astronomy tests, Hagrid is fired by Professor Umbridge and several other Ministry personnel. Professor McGonagall is stunned but these people trying to stop them from tussling with Hagrid. Later that night Harry dreams that Sirius is being tortured by Voldemort. Ron, Hermione, Neville and Luna help Harry to breaking into Professor Umbridge's office where he attempt to contact Sirius. Kreacher tells Harry that Sirius is at the Department of Mysteries. They are found out by Professor Umbridge and Hermione tricks Umbridge into taking her and Harry into the Forbidden Forest. They lead her into the Forbidden Forest where she insults the centaurs and they take her away. Grawp unwittingly rescues Harry and Hermione from them. Ron, Ginny, Neville and Luna find Harry and Hermione in the forest and they insist on helping them find Sirius. They ride on the thestrals (which can only be seen by those who have seen death) to get them to London. Once there they get into the Ministry of Magic through the telephone booth that Arthur had used previously. They go to the Department of Mysteries that Harry had been dreaming about so much and discover a prophecy containing Harry's name. Lucius Malfoy, Bellatrix Lestrange (who tortured and caused mental illness in Neville's parents) and more Death Eaters show up. They try to get the prophecy from Harry. A struggle ensues in which several people get hurt. The prophecy is broken but no one can hear it over the battle. Lupin, Sirius and other members of the Order of the Phoenix show up and fight the Death Eaters. Dumbledore arrives and rounds up all of the Death Eaters but not before Sirius, battling with Bellatrix, falls through the Veil and dies. Harry chases after Bellatrix and tries a Cruciatrix on her. He learns he is incapable of an Unforgivable spell since he doesn't "mean" it, doesn't take pleasure in it. Voldemort arrives and Dumbledore saves Harry from the Avada Kadavra. Harry get possessed but wakes up after Voldemort is driven away. Mr Fudge and some other Ministry members witness Voldemort. Dumbledore reinstates himself as Headmaster of Hogwart's with little problems from Mr Fudge.
Harry gets angry about the death of Sirius and Dumbledore decides to tell him everything. He has to return to the Dursleys every year because of the protection of blood relations against Voldemort. The prophecy in which Voldemort marked him as his equal and that either Harry will kill Voldemort or Voldemort will kill him. Neville is revealed as another person that the prophecy could have been about. Professor Dumbledore also admits that his feelings caused him to hold the truth from Harry for too long. Ron, Hermione and the others all recover from their injuries and Mr Fudge admits publicly that Voldemort has returned. Harry asks Nearly Headless Nick if Sirius would come back as a ghost but Nick says that only wizards who cannot stand the fact of dying can come back as ghosts and that Sirius would not be coming back. After term is over, Harry rides home with Ron, Hermione, Neville and Luna. Moody, Lupin and Tonks meet him at the station. They warn the Dursleys to be good to him during the summer and that if they do not hear from him for more than three days in a row, someone will come to check on him.
Extract (may contain spoilers)