The story
After the events of the Triwizard Tournament, Harry expects the wizarding world to prepare for danger. Instead, official institutions deny his account and the Ministry of Magic tries to control both public information and life inside Hogwarts.
Dolores Umbridge's arrival turns the school into a political battleground. In response, Harry and his friends create Dumbledore's Army, a student group formed to practise the defensive magic they are being prevented from learning. The book also opens the Ministry of Magic as a major location and explores the Order of the Phoenix as an organised resistance network.
This is the series at its most institutional: newspapers, government, education and public opinion all affect the conflict. It explains much of the political structure behind the later books and gives several recurring locations their full narrative importance.