Chapter 2 - "Aunt Marge's Big Mistake" - In which Aunt Marge comes to visit and her taunting drives Harry to inflate her, then gather his belongings and leave Privet Drive without permission to visit Hogsmeade.
Homework at Midnight
[Harry] opens his last gift, from [Hagrid], which turns out to be the [Monster Book of Monsters]. He manages though, despite the book's attempts to bite him, to get a belt wrapped around it to keep himself safe.
The final letter is from [Hogwarts], with [Harry]'s booklist for his third year. It also includes a permission form for [Hogsmeade], which makes Harry dread trying to get the [Dursleys] to sign it - but nevertheless he heads off to bed, glad that it's his birthday.
Horrible Birthday with Aunt Marge
At breakfast, [Harry] arrives to find the [Dursleys] watching their new TV, and hearing an announcement that a man named [Black] has escaped prison and is on the loose, prompting [Uncle Vernon] to complain that more convicts should be hanged.
[Harry] unexpectedly learns that [Aunt Marge] is coming for a visit, and thinks back to all the times she has tormented him. [Vernon] takes advantage of the opportunity to threaten Harry to behave before heading out the door.
Following [Uncle Vernon] out the door, [Harry] makes him promise that if he behaves while [Marge] is visiting, he'll sign Harry's [Hogsmeade] form.
[Harry] heads up to his room to clean up so he can pretend he's a [Muggle], hiding his birthday presents and sending [Hedwig] off to stay with the [Weasleys].
Aunt Marge's Explosive Week
[Aunt Marge] arrives and wastes no time in doting on [Dudley] and criticizing [Harry], though he pleases her somewhat when he says he's beaten regularly at his supposed school, [St. Brutus's].
The week continues on and one afternoon over lunch, when [Aunt Marge] criticizes [Harry]'s mother, [Harry] accidentally explodes her wineglass. He quickly escapes the room, worried about his possible expulsion from [Hogwarts].
[Harry] makes it through the rest of the week with [Aunt Marge] without incident, but on the final night of her stay, she criticizes Harry's parents loudly. Harry first defends them, then accidentally blows Marge up like a balloon and runs out of the house before [Vernon] can do anything to him.