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
[Harry], now stuck in his room, quickly writes notes to [Ron], [Hermione], and [Sirius], and sends them off rudely with [Hedwig] before rolling over to fall asleep.
For three days [Harry] waits in his bedroom, hearing nothing and wondering about where [Hedwig] is, and what will happen with his upcoming hearing at the [Ministry of Magic].
[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.
[Tonks] escorts [Harry] to his bedroom to help him pack things up. They talk briefly about her being a [Metamorphmagus] and an [Auror], and then head back downstairs to prepare to depart.
Back in the kitchen, [Moody] [Disillusions] [Harry] and the group gathers together, spots their signal, and takes off.
[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.