Spoilers ahead for the earlier books.
It opens with Harry being sulky about being kept in the dark. (He eavesdrops on the evening news in hopes of hearing stories that will hint of Voldemort's reappearance.) He and Dudley are attacked by dementors, and he has to use magic just to stay alive. Whereupon he finds himself being threatened with loss of his wand, expulsion from Hogwarts, and being thrown out the house -- while getting owls that he has to stay put. One finally persuaded his aunt to insist that he stay for a time.
And when wizards show up to sweep him away, he meets up again with Sirius and the Weasleys but finds himself little more informed. Fudge and the Ministry are doing their best to stick to the story that Voldemort didn't return, by smearing Dumbledore and Harry. He survives the hearing with an acquittal, but it was an obvious attempt to railroad him. And Sirius Black is miserable because he is effectively confined to home lest he be caught.
And they return to school, only to find the Ministry of Magic is trying to assert control there. It involves no practice of Defense of the Dark Arts in class, arbitrary orders, an -- unusual student named Luna Lovegood, an interview, a swamp, Hagrid's returning late, and much more.
I think this is my least favorite book in the series. For one thing, Harry is sulky a lot; very accurate for that age (my mother dislikes this book because she got enough of that attitude teaching high school chemistry) but not much fun. Also it's the darkest of the books because of the level of paranoia, with all the denial going on, and I'm not much of a fan of what happened with Sirius.
Also, when Cho talks about Cedric's death and Harry heavily assures her that knowing all this didn't save him, I thought[Spoiler (click to open)]if you two had not merely gone wands out but adopted a policy of stun first and sort out later you could have both lived, and captured Peter Pettigrew and quite possibly Voldemort while he was still helpless. Now, they had been caught by surprise, so it's not unnatural for them to not think of it at the time, but perhaps he might have thought of it since.