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.
Aunt Marge's Explosive Week
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.