Harry Potter, Ron Weasley and Hermione Granger enter their fifth year at Hogwarts School of Witchcraft and Wizardry. Much of the wizarding community has shunned Harry and Albus Dumbledore, Hogwarts headmaster, who claim that Lord Voldemort has, after 14 years, regained a body and been restored to full power, and are instead influenced by the propaganda of The Daily Prophet and the ignorance of the Minister for Magic, Cornelius Fudge. Fudge appoints a new Hogwarts Defence Against the Dark Arts teacher, Dolores Umbridge, who begins to take over Hogwarts through the support of the Ministry.

Following the Ministry line, Umbridge refuses to acknowledge that Voldemort has returned, and when Harry insists that he has, she gives him detention during which he is required to write lines with a quill that causes the words to be scratched into the skin of his hand. Umbridge's teaching is inadequate, consisting of memorisation of the textbook only instead of any practical instruction in defensive spells. In order to continue practical Defence Against Dark Arts training, twenty-seven students form "Dumbledore's Army" (the D.A.), an underground organisation in which Harry teaches defensive magic.