Chapter 3 - "The Advance Guard" - In which Harry is rescued from number four Privet Drive, Remus Lupin and Alastor "Mad-Eye" Moody are reintroduced, and Harry meets Nymphadora Tonks for the first time.
Harry Explains About the Dementors to the Dursleys
With owls constantly swooping in to deliver messages to Harry - first expelling him from Hogwarts, then telling him to stay put while things get sorted out - he explains the [dementor] attack to [Vernon] and [Petunia]. Petunia, though, betrays that she knows more about magic than Harry thinks, and when Vernon tries to kick him out of the house, she instead allows him to stay.
Harry Alone in his Room
[Vernon] shows up to tell Harry that he, [Petunia], and [Dudley] are leaving and locking him into his bedroom. He listens as they leave, and then lies miserably in his room in the dark, waiting for nothing in particular.
Harry is Escorted Back to the Wizarding World
Unexpectedly the door to Harry's bedroom opens, and though he's suspicious at first, he soon finds Moody, Lupin, and Tonks in his hallway, and follows them to the kitchen in disbelief. There he finds a large number of wizards, all come to escort him away from Privet Drive, and after being warned by Moody not to discuss Voldemort, runs back upstairs to quickly pack.
Harry and his group escort take off to fly to [London], with Moody in the lead. Despite the cold air and Moody's attempts to double back and throw off any pursuers, the group finally lands in a square. There Moody thrusts a piece of paper into Harry's hands, identifying their location as Grimmauld Place, in [London].
Chapter 4 - "Grimmauld Place" - In which Harry is reunited with Ron and Hermione, learns of the Order of the Phoenix, and is reunited with his godfather, Sirius Black.