Hearing that Harry is struggling out in Little Whinging, the Order of the Phoenix turns up at the Dursleys' house to take Harry to their headquarters. The Order of the Phoenix is the group of wizards who resisted Voldemort during the first war against him fifteen years ago. They have reassembled thanks to Harry's news at the that Voldemort is rising again. They bring Harry to Number Twelve, Grimmauld Place, a dark and gloomy townhouse in a Muggle section of London. Harry is shocked not only to find his friends Ron and Hermione waiting for him, but also his godfather, Sirius Black.
We start Harry Potter and the Order of the Phoenix back in Little Whinging, Surrey, where Harry is stuck for yet another summer with his awful relatives, the Dursleys. The Dursleys keep up their usual nonsense – wizards are freaks, Harry is a wizard, therefore Harry is a freak – but Harry seems to have changed. He simmers with fury all the time. None of his friends are writing to him about the war with Voldemort, and he feels isolated and resentful. Harry starts provoking fights with his dumb cousin Dudley. He wants to have a target for his rage. But in the middle of one of these confrontations with Dudley, something completely bizarre happens. In Little Whinging, Surrey, two dementors show up!
In fact, Sirius and Harry are experiencing similar feelings of grim misery right now: Sirius is stuck inside all the time, unable to do anything substantial to help the cause against Voldemort. And Harry feels left out of the planning of the war, even though he has faced Voldemort four times and survived! Both Harry and Sirius feel resentful and isolated from their friends, which makes the tone of Harry Potter and the Order of the Phoenix much darker than previous installments in the Harry Potter series.
Well, one night, Harry sees something extremely troubling: at the end of the corridor he keeps revisiting in his nightmares, he sees Voldemort torturing Sirius. Harry assumes that Voldemort must have Sirius, and he decides to ride to the rescue. Without Professors Dumbledore or McGonagall at Hogwarts (since McGonagall was injured trying to defend Hagrid from Aurors ordered to remove him Hogwarts by Professor Umbridge), he tries to pass the message on to Professor Snape (who is a member of the Order of the Phoenix). But Harry doesn't like or trust Professor Snape enough to believe that the man is going to help Sirius.