This book is about Harry Potter's fifth year at Hogwarts. In this book, he runs into more challenges, and one of the main ones is his OWL exams for his 5th year.
The story begins during the summer, when he was living at the Dursley's house, his aunt's family's house. Harry and his cousin, Dudley were attacked by Demontors when they were walking home, and Harry performed the Patronus charm to get them away, he succeeds. However, when the Ministry of Magic learns about this, letters were sent to the Dursleys' house, and one of them was that Harry had been expelled. But fortunately, after a hearing at his trial from the Ministry of Magic Harry was cleared of all charges and wan't expelled.
After the summer Harry returns to Hogwarts for his 5th year. They have a new Defense Against the Dark Arts teacher, her name is Dolores Umbridge. Umbridge becomes an unpopular teacher at Hogwarts very quickly. She didn't let the students perform any magic in class, she gave detentions to Harry after he told the truth, and because she is the high inquisitor, she inspected teachers and their classes and teachers didn't like it. Because of her not allowing any magic to be performed in class, Harry and some of his friends start a secret Defense Against the Dark Arts group, where they practice magic and how to defend themselves.
I think this is a good book, and I would suggest this book to people who are looking for a god book to read, especially if you've read the first 4 books.