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 they need to learn the subject themselves. Harry is surprised when she proposes him as a teacher, but she reminds him that his adventures of the last four years at Hogwarts have actually made him an authority on the subject. Several other students from Gryffindor, Ravenclaw, and Hufflepuff likewise sign up to the clandestine group, named "Dumbledore's Army" in mockery of Fudge’s paranoia.
Under Harry's tutoring, the group learns defensive magic. The last meeting before Christmas, Cho Chang stays after, and Harry, hoping to receive a Merry Christmas, gets an even bigger surprise -- a kiss. When they return to Hogwarts after Christmas break, Harry asks her to go with him to Hogsmeade on Valentine's Day. However, while they are there, Harry mistakenly makes Cho think he likes Hermione. Cho breaks up with him.
To combat the Ministry's smear campaign against Harry and Dumbledore, Hermione blackmails yellow journalist Rita Skeeter into writing an interview with Harry about his witnessing Voldemort's return. Ravenclaw student Luna Lovegood's father publishes the story in his magazine, The Quibbler. Furious, Umbridge bans the Quibbler from the school, but the story spreads rapidly, garnering support for Harry. Harry and Cho get back together.
Midway through the year, the school is alarmed to hear news of a mass prison break from Azkaban Prison, 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, while the D.A. members are inspired to work even harder.
Eventually Umbridge's Inquisitorial Squad, comprising mostly of Slytherin students, uncover the D.A. meetings, helped by a member of the D.A. named Marietta, who told Umbridge about the meetings. To protect Harry and other students from reprisals, Dumbledore claims that he organized the group. Confronted by Fudge, Percy Weasley, Umbridge and Aurors (John Dawlish and Kingsley Shacklebolt), Dumbledore easily overpowers them and is whisked away by his phoenix, Fawkes. Cho, who is Marietta's friend, gets mad at Harry for accusing Marietta and breaks up with him again.
Umbridge becomes Headmistress and enacts even stricter rules, firing Hagrid. The disenchanted Weasley twins revolt, unleashing relentless magical chaos throughout the school, while the staff purposely does nothing to help Umbridge regain control. The twins are caught, but summoning their confiscated brooms, they fly away, leaving Hogwarts to open their own joke shop.
Throughout the year, Harry has disturbing dreams about running down a hallway and attempting to open a door in the Ministry's Department of Mysteries. On Christmas Eve, he dreams he is a snake attacking Ron's father. Mr. Weasley is indeed found injured at the Ministry, suffering from severe venomous snakebites, causing Harry to fear that Voldemort is possessing him. In response, Dumbledore has Severus Snape teach Harry Occlumency to block his mind from intrusion, but their mutual animosity ends their lessons prematurely. In the course of these lessons, Harry inadvertently sees one of Snape’s memories from his school years, and is shocked to see his father James Potter bullying Snape and acting as arrogantly as Snape always said he did.