Harry Potter and the Order of the Phoenix

This novel begins when Harry and his cousin, Dudley, are attacked by dementors. Harry uses magic to fight them off, and must attend a disciplinary hearing for it. It quickly becomes apparent that the hearing is a show trial, and despite the best attempts of the Ministry of Magic, he is cleared of all charges. In response to Voldemort's reappearance, Dumbledore re-activates the Order of the Phoenix, a secret society which works to defeat Voldemort's minions and protect Voldemort's targets, including Harry. Despite Harry's description of Voldemort's recent activities, the Ministry of Magic and many others in the magical world refuse to believe that Voldemort has returned.
In an attempt to enforce its version of school curriculum, the Ministry appoints Dolores Umbridge as the new High Inquisitor of Hogwarts. She transforms the school into a quasi-dictatorial regime and refuses to allow the students to learn ways to defend themselves against dark magic. Harry forms a secret study group and begins to teach his classmates the higher-level skills he has learned. The novel introduces Harry to Luna Lovegood, an airy young witch with a tendency to believe in oddball conspiracy theories. Moreover, it reveals an important prophecy concerning Harry and Voldemort. Harry also discovers that he and Voldemort have a telepathic connection, allowing Harry to view some of Voldemort's actions. In the novel's climax, Harry and his school friends face off against Voldemort's Death Eaters. The timely arrival of members of the Order of the Phoenix saves the children's lives, but Sirius Black, Harry's godfather, is murdered by Bellatrix Lestrange. Many Death Eaters are captured and, most importantly, the return of Voldemort is confirmed within the magical world.