Several weeks into the summer, Harry Potter has heard nothing from his. One night, walking down a street his cousin, they were attacked by Dementors. Harry did a Patronous Charm and learns that Arabella Figg is a Squib, has been keeping an eye on him on Dumbedore’s orders. He receives a notice of expulsion from Hogwarts for using magic outside school. Someone from the Order of Phoenix then takes him to the headquarters.
A few days later, Arthur escorts Harry to his hearing, which Fudge has done everything in his power to slant against him. But testimony from Dumbledore and Mrs. Figg confirms the presence of the Dementors, and Harry is found to have acted in self-defense.
Ron and Hermione are made prefects, but not him. At the welcome feast, the learn that their new Defense Against the Dark Arts teacher at Hogwarts is Dolores Umbridge. A short time later, she is appointed as High Inquisitor, with the power to evaluate and dismiss other teachers, and impose strict rules and regulations on students.
Hagrid was absent for half the year, because of a diplomatic mission to the giants on Dumbledore’s orders, to try to dissuade them from joining Voldemort. His mission was a failure. Because Umbridge’s lessons are useless both for passing their O.W.L.s and for preparing for the very real threat of Voldemort, Hermione decides that the students need to learn the subject themselves. Harry is surprised when she proposes him as a teacher. Several other students from Gryffindor, Ravenclaw, and Hufflepuff likewise sign up to the clandestine group, named "Dumbledore's Army". Under Harry's tutoring, the group learns defensive magic.
Midway through the year, the school is alarmed to hear news of a mass prison break from Azkaban, when ten of Voldemort’s most powerful Death Eaters escape. Many wizards are not satisfied with the Ministry’s official explanation, and begin to come around to Dumbledore and Harry’s claims.
Slytherin students, uncover the D.A. meetings, helped by a member of the D.A. named
In the middle of his last O.W.L. exam, Harry has a vision of Sirius being tortured at the Department of Mysteries, although Hermione suspects it may be a trap. Harry, with help from various members of Dumbledore's Army, attempts to contact Sirius, but he is caught.
They then went to the Ministry of Magic to seek Sirius. They meet up with the Death Eaters, and Harry and his friends heroically defend themselves, putting up a far tougher fight than the dark wizards expected, but are outmatched. As they are nearly defeated, members of the Order arrive, including Sirius. During the ensuing battle, the glass sphere that Voldemort was seeking is accidentally dropped and shatters, and the record is lost. Sirius is died in the fight. The Death Eaters were captured except for Bellatrix.
The one with the power to vanquish the Dark Lord approaches... born to those who have thrice defied him, born as the seventh month dies... and the Dark Lord will mark him as equal, but he will have power the Dark Lord knows not... and either must die at the hand of the other for neither can live while the other survives... the one with the power to vanquish the Dark Lord will be born as the seventh month dies...
Taken from Harry Potter and the Order of the Phoenix by J.K. Rowling