We start Harry Potter and the Order of the Phoenix back in Little Whinging, Surrey, where Harry is stuck for yet another summer with his awful relatives, the Dursleys. The Dursleys keep up their usual nonsense – wizards are freaks, Harry is a wizard, therefore Harry is a freak – but Harry seems to have changed. He simmers with fury all the time. None of his friends are writing to him about the war with Voldemort, and he feels isolated and resentful. Harry starts provoking fights with his dumb cousin Dudley. He wants to have a target for his rage. But in the middle of one of these confrontations with Dudley, something completely bizarre happens. In Little Whinging, Surrey, two dementors show up!
Dementors are the guards of the wizard prison Azkaban. They are also magical creatures that feed off fear. If they get the chance, they'll fasten their mouths over their victims' lips and suck the souls right from their bodies. Even though Harry is fifteen and underage according to wizarding law, so he isn't supposed to use magic outside of Hogwarts. But he can't let the dementors suck his cousin's soul out. He casts a Patronus charm to repel the dementors. The Ministry of Magic immediately sends Harry a notice scheduling a hearing. If Harry is found guilty of the charge of casting a spell as an underage wizard (especially in a Muggle – non-wizarding – area), he'll get expelled from Hogwarts.
Hearing that Harry is struggling out in Little Whinging, the Order of the Phoenix turns up at the Dursleys' house to take Harry to their headquarters. The Order of the Phoenix is the group of wizards who resisted Voldemort during the first war against him fifteen years ago. They have reassembled thanks to Harry's news at the end of Book 4 that Voldemort is rising again. They bring Harry to Number Twelve, Grimmauld Place, a dark and gloomy townhouse in a Muggle section of London. Harry is shocked not only to find his friends Ron and Hermione waiting for him, but also his godfather, Sirius Black.
In fact, the house belongs to Sirius. It has been in the Black family for ages. The Black family is a pureblood line of Dark wizards: Sirius was the first Black to be in Hogwarts' Gryffindor House, like, ever. Sirius's cousin, Bellatrix Lestrange, has become one of Voldemort's most powerful Death Eaters. And his other cousin Narcissa is mother to Harry's arch-nemesis at school, Draco Malfoy. Sirius has given the Grimmauld Place house to the Order because it's the biggest contribution he can make right now. Since Sirius is a convicted murderer (though falsely accused), he can't go out in public. He is also an Animagus who transforms into a dog (a form he calls "Snuffles"), but Voldemort is aware of his dog form, so Sirius can't use that either. He is trapped in Number Twelve, Grimmauld Place, surrounded by memories of his unhappy childhood.
In fact, Sirius and Harry are experiencing similar feelings of grim misery right now: Sirius is stuck inside all the time, unable to do anything substantial to help the cause against Voldemort. And Harry feels left out of the planning of the war, even though he has faced Voldemort four times and survived! Both Harry and Sirius feel resentful and isolated from their friends, which makes the tone of Harry Potter and the Order of the Phoenix much darker than previous installments in the Harry Potter series.
Eventually, Harry goes to his hearing. Thanks to the testimony of Arabella Figg, a Squib (non-magical person with magical parents) who has been living next to the Dursleys and watching out for Harry on Professor Dumbledore's orders, Harry can prove that there were dementors in Little Whinging and he was acting in self-defense. While Professor Dumbledore helps to prove Harry's innocence, Harry is angry that Dumbledore refuses to speak to or even look at Harry. Harry can't understand why Professor Dumbledore seems to have turned his back on Harry, so Harry decides to turn his back on Professor Dumbledore. He refuses to tell Dumbledore about any of the things he experiences during the school year, and in fact, the two don't speak properly until the very end of the novel.
This whole hearing business sets up one of the major plot lines of the novel: the Minister for Magic, Cornelius Fudge, has decided to discredit Harry as much as possible. The Minister does not want to acknowledge that Voldemort has come back, so he insists on this hearing to try and get Harry kicked out of school. What's more, Fudge pressures the Daily Prophet newspaper to report that Harry is dangerously unstable. He also sends a Ministry representative, Dolores Umbridge, to Hogwarts as Defense Against the Dark Arts instructor. She is supposed to be teaching, but in fact, she's there as a spy for Fudge. She is keeping close watch on Harry and on Professor Dumbledore, whom Fudge suspects of trying to steal his post as Minister for Magic.
Professor Umbridge is a power-hungry monster who spends her time at Hogwarts tightening controls on every aspect of Hogwarts life: she gives Harry detentions in which she forces him to write with a Blood Quill that cuts into his hand every time he uses it. She gets herself appointed Hogwarts High Inquisitor, which gives her the authority to observe other professors and decide if they are doing their jobs properly. She forbids students from meeting in groups of three or more without official permission. And she appoints all of the worst bullies in the school, including Draco Malfoy and Pansy Parkinson, to an Inquisitorial Squad with powers to punish their fellow students whenever they see fit. Professor Umbridge's power trip makes her incredibly unpopular with students and teachers alike, but with the backing of the Ministry of Magic, it seems impossible to curb her growing power.
Harry finds the pressure of dealing with Professor Umbridge unbearable. Not only does half the wizarding world think he's making up stories about Voldemort thanks to the Daily Prophet, but Professor Umbridge tells him to his face and in front of the student body that he is lying. The only relief Harry gets from all of this stress is from Dumbledore's Army. This is a student group suggested by Hermione; it's strictly forbidden by Umbridge, but they go ahead and start meeting in secret anyway.
Because Professor Umbridge refuses to allow them to practice defensive spells during Defense Against the Dark Arts, this inter-House group gathers together to study things like the Disarming Spell and the Patronus Charm with Harry as their leader. They call themselves Dumbledore's Army because they know that that is what the Ministry is most afraid of: that Dumbledore is using Hogwarts to train powerful wizards who are loyal to him. The name is a joke, though: this group meets without the direct authority of Dumbledore. Harry is proud of all of the progress his students are making, and he is delighted to resist Professor Umbridge right under her evil nose.
At least Harry has the D.A. to look forward to; otherwise, his life continues to suck for most of the fifth book. Because Professor Umbridge has it in for him, she gives him a life-long ban from playing Quidditch. What's worse, Harry has been having terrible nightmares in which he seems to be looking through the eyes of Voldemort. Things come to a head in Chapter 21, when Harry sees Voldemort's huge snake attacking Mr. Weasley in the basement of the Department of Mysteries. Harry rushes to Professors Dumbledore and McGonagall to tell them that Mr. Weasley has been attacked. As soon as Professor Dumbledore hears what Harry has been seeing, he tells Harry to start studying Occlumency.
Occlumency is a little-known field of magic that is supposed to protect a wizard's mind from magical scanning. And who's the best Occlumens at Hogwarts besides Professor Dumbledore himself? That would be Professor Snape, of course – Harry's most-hated teacher. Professor Snape and Harry start their lessons in secret, on Dumbledore's orders. But the lessons leave Harry feeling like his mind is even more open than it was naturally. He is dreaming more vividly every night. And Professor Snape is also catching glimpses of Harry's most painful and embarrassing memories, things Harry doesn't want anyone to see, let alone Professor Snape.
One day, Harry happens to be in Professor Snape's office alone and he decides to get a bit of revenge. He looks into Professor Snape's Pensieve, which is a storage place for memories. In it, he sees Professor Snape's worst memory: fifteen-year-old Snape getting brutally bullied by Harry's father, James, and his three best friends, Sirius, Remus, and Wormtail. So not only has Harry started to resent Professor Dumbledore, but now he is starting to wonder if his own father and godfather are really good people. All of Harry's heroes and idols seem to be falling off their pedestals.
One of the members of the D.A. rats out her fellow members to Professor Umbridge, who comes down on their meeting like a ton of bricks. She finds the list of members under the joke heading, "Dumbledore's Army," and she and Minister Fudge take it very seriously. They use it as evidence to try and arrest Professor Dumbledore, but he leaves Hogwarts on his own power. This leaves Harry without any obvious mentors or allies at school.
Of course, all of these problems – Sirius's isolation, Harry's anger, and Harry's lack of trusted authority figures at Hogwarts – come to a head in the final chapters of the book. Sirius has grown so reckless after a year trapped in Number Twelve, Grimmauld Place that he will jump at anything to get out of the house. And Harry has grown so sure that Professor Dumbledore has been deliberately cutting him off from necessary information that Harry has been trying desperately to see things through his increasingly frequent nightmares.
Well, one night, Harry sees something extremely troubling: at the end of the corridor he keeps revisiting in his nightmares, he sees Voldemort torturing Sirius. Harry assumes that Voldemort must have Sirius, and he decides to ride to the rescue. Without Professors Dumbledore or McGonagall at Hogwarts (since McGonagall was injured trying to defend Hagrid from Aurors ordered to remove him Hogwarts by Professor Umbridge), he tries to pass the message on to Professor Snape (who is a member of the Order of the Phoenix). But Harry doesn't like or trust Professor Snape enough to believe that the man is going to help Sirius.
So Harry rushes to the Ministry of Magic accompanied by a few of his friends who are also members of the D.A.: Ron, Hermione, Ginny, Neville, and Luna Lovegood. Once they get to the basement of the Department of Mysteries, they find a room full of glass balls that Harry has been seeing in his dreams. He searches everywhere, but there is no Sirius. In fact, the dreams have been sent by Voldemort all along. Voldemort has lured Harry to the Department of Secrets so that Harry can pull out a prophecy (in the form of a glass ball) kept there for him. Harry finds the prophecy, but he and his friends are ambushed by a circle of Death Eaters.
In the fight that ensues, Ginny breaks her ankle, Hermione gets horrible internal injuries, Ron grapples with a giant brain, Luna Lovegood gets Stunned, and Neville Longbottom breaks his nose and gets Crucio-d by the same Death Eater who tortured his parents to insanity. Harry has led his friends right into a trap and they are paying the price. But worst of all is that Sirius Black and the rest of the Order of the Phoenix ride to the rescue. Sirius slips through an apparently empty archway in the Department of Mysteries and never reappears. He is dead, killed recklessly trying to save Harry.
Professor Dumbledore arrives at the Department of Mysteries around the same time that Voldemort does. The two have a showdown that ends in Voldemort trying to possess Harry to force Dumbledore to kill the boy. But Harry's heart is too full of emotion for Voldemort to keep control, and he scurries off. But Voldemort doesn't disappear before Cornelius Fudge and a whole bunch of Ministry employees see old Voldy with their own eyes. So Fudge can no longer deny that Harry and Professor Dumbledore have been telling the truth and Voldemort is back. He removes Professor Umbridge from power and places Professor Dumbledore as Headmaster of Hogwarts once again.
Back at Hogwarts, Professor Dumbledore apologizes to Harry for keeping him in the dark for all of this year. Dumbledore explains that he was trying to protect Harry, the same way he tried to protect Sirius by leaving him in Number Twelve, Grimmauld Place. Now, Professor Dumbledore can see that leaving an active man to rot in a house by himself may not have been the best plan. But he had the best of intentions. Harry rages at Professor Dumbledore, but his anger is also a cover for his own guilt: why did he have to fall for Voldemort's trap and blunder into the Department of Mysteries in the first place?
Professor Dumbledore explains the prophecy to Harry. It was made in the year Harry was born, and it states that a boy born at the end of July to parents who have defied Voldemort three times will arrive with power that Voldemort doesn't have. There were two candidates to fill the prophecy, Harry and Neville Longbottom. But by attacking Harry when he was a baby and leaving Harry with his curse scar and magical connection to the Dark Lord, it's Voldemort who selected Harry to be the boy of prophecy. Now Harry must kill Voldemort or else be killed by him.
So now Harry knows why Voldemort has been coming after him: because Voldemort believes that Harry continues to present a danger to his rise to power. And it's Harry's fate to keep facing Voldemort until one or the other of them is dead. With this understanding of his fate, and with his guilt and sorrow over Sirius's death, Harry feels much less anger towards his friends. They all know that war is coming soon, and that the time for emo brooding is over: Harry has to start acting like an adult to face the challenges that are coming his way.