Harry Potter is having a very agitating summer: not only is he bracing himself for the catastrophe that is sure to accompany Voldemort's return, but he has also been stuck in Privet Drive for a month, with little contact from the wizarding world. But when Harry and his cousin Dudley are attacked by Dementors in Little Whinging, Harry fends them agic outside school. Several more letters arrive in quick succession: Arthur Weasley and Sirius Black instruct Harry to remain in the house, while another overturns his expulsion and orders him to appear at a hearing at the Ministry of Magic.
Several Order of the Phoenix members arrive and escort Harry to their secret headquarters at the Black family home, 12 Grimmauld Place in London. The Weasleys, Hermione, and Harry’s godfather Sirius Black are there. Harry learns that Voldemort is building an army and is attempting to retrieve a "weapon". Harry is cleared of all charges at his hearing.
On the train to Hogwarts, Harry, Ron, and Hermione are introduced to Luna Lovegood, a fourth-year Ravenclaw, and a girl who is immediately discovered to be extremely odd, frequently talking about things like Blibbering Humdingers and Crumple-horned Snorkacks.
Ron is chosen as the new Keeper for the Gryffindor Quidditch team, and, although he is fairly talented on the field, he tends to get nervous and lose his head when people are watching him.
The students return to Hogwarts, where Dolores Umbridge, Senior Undersecretary to the Minister, has been appointed by the Ministry as the new Defence Against the Dark Arts teacher. She refuses to teach students real defense methods and only uses textbooks to teach Ministry-approved theory. It is soon clear she is only there to spy on and take control of the school and is soon appointed High Inquisitor, arbitrarily imposing rules and regulations. She also harbours an intense dislike for half-breeds, such as centaurs, werewolves, and similar creatures. She considers Rubeus Hagrid (a half-giant) and Sybill Trelawney incompetent, and fires Trelawney. Dumbledore, much to Umbridge's fury, appoints Firenze, the centaur we meet in book one, as the new Divination teacher.
Harry has been having strange dreams about running down a hallway and attempting to open a door in the Ministry of Magic's Department of Mysteries. He also dreams he is a snake attacking Ron's father, Arthur Weasley. Arthur is indeed found injured at the Ministry, suffering from a severe venomous snake bite. Harry fears he is being possessed by Voldemort to do his bidding. In response, Dumbledore has Severus Snape teach Harry Occlumency to block his mind from intrusion, but their mutual dislike ends their lessons prematurely.
Hermione blackmails journalist Rita Skeeter into writing a favourable newspaper article about how Harry witnessed Voldemort's return. Luna Lovegood persuades her father to publish the story in his paper The Quibbler. Furious, Umbridge bans the tabloid at the school, but the story spreads rapidly, gathering support for Harry.
Hermione convinces Harry to teach students Defence Against the Dark Arts. They name their clandestine group "Dumbledore's Army", or D.A. for short, to mock the Ministry of Magic, which fears that Dumbledore is creating a secret wizard army. When Umbridge uncovers the group's meetings, Dumbledore claims that he organized the group, to divert attention from Harry. Confronted by two Aurors (Dawlish and Shacklebolt), Minister Fudge, Percy Weasley, and Umbridge, Dumbledore easily disables them and is spectacularly whisked away by his phoenix, Fawkes. Umbridge is appointed Headmistress and imposes even tighter controls on the students. Fed up, the Weasley twins instigate a revolt, causing mayhem throughout the school while the staff pointedly do nothing to help Umbridge regain control.
The final match to decide which house will take the Quidditch Cup is Gryffindor versus Ravenclaw. Harry and Hermione are in the stands to watch, but they are asked by Hagrid to come with him into the Forbidden Forest while no one is watching. They follow him, and when they reach their destination, Hagrid tells them about how he brought his younger half-brother, Grawp, back from the the mountains where the giants live, and is now attempting to teach him to speak English. He asks Harry and Hermione if they would continue these efforts in case he is sacked by Umbridge. They reluctantly agree. Though they see none of it, Ron helps Gryffindor to win the Quidditch cup.
During the Astronomy exam, which takes place at night on top of a tower, Harry, Ron, and Hermione see Umbridge and several others attack Hagrid, and drive him from his hut.
During one of his O.W.L. exams, Harry has another vision: that Sirius is being tortured at the Department of Mysteries. Unaware it is a ruse, he desperately attempts to contact Sirius via the Floo network in Umbridge's office fireplace, but he is caught. Umbridge reveals it was she who sent the Dementors to attack Harry during the summer. She is about to use the Cruciatus Curse on him, when Hermione pretends to break down into tears and persuades Umbridge to believe that Dumbledore has hidden a powerful weapon in the Forbidden Forest. She leads Harry and Umbridge into the forest, where they encounter centaurs. Umbridge foolishly insults them and, angered, a centaur picks up Umbridge and carries her off screaming into the woods. When Hagrid's giant half-brother, Grawp crashes onto the scene, Hermione and Harry escape amid the chaos.
Harry, Ron, and Hermione, along with Luna, Ginny, and Neville fly to the Ministry of Magic on the school's Thestrals, unaware they are being lured into a trap. When they arrive at the Department of Mysteries, they are ambushed by Death Eaters. Voldemort is seeking a prophecy contained in a glass sphere there and needs Harry to retrieve it for him.
The students heroically fight the Death Eaters, but they are outmatched. Just as they are nearly defeated, Order members arrive. During the ensuing battle, the glass sphere is shattered and the prophecy lost. Sirius is blasted with a spell by his Death Eater cousin, Bellatrix Lestrange and, falling backwards, disappears through a mysterious veiled archway. Lupin reveals to Harry that Sirius is dead. Dumbledore arrives and the Death Eaters, except for Bellatrix, are captured. Lord Voldemort appears and duels with Dumbledore as Ministry of Magic employees arrive in time to see the Dark Lord before he Disapparates, taking Bellatrix with him. Cornelius Fudge finally admits Voldemort has returned and Harry's interview with Rita Skeeter is reprinted in the Daily Prophet.
Later, in his office, Dumbledore apologizes to Harry for neglecting him over the past year. He reveals the lost prophecy's content: either Harry or Voldemort must die at the hand of the other, for neither can live while the other survives.
At the end of the year, Harry returns to the Dursleys' for another summer, deeply grieving the loss of his godfather.
OK, I did take this right from Wikipedia, but I feel alright putting it as my answer, seeing that I wrote a lot of it anyways. Hope it helps!