Harry comes down for breakfast the next morning. The Muggle news is reporting the escape of a dangerous convict named Black. Uncle Vernon has worse news for Harry: Aunt Marge (Dudley's aunt, not actually Harry's) is coming around for the next week. Uncle Vernon agrees to sign his Hogsmeade if Harry spends the week pretending he attends "St. Brutus's Secure Centre for Incurably Criminal Boys".
Marge arrives and begins her stay. She is even more boorish and bigoted than her brother Vernon. And whereas the Dursleys normally prefer Harry to stay out of the way, Marge likes to have him around so that she can taunt and insult him. At one point, she suggests that Harry has turned out badly because of who his parents were. At that very moment, the wine glass in her hand shatters. Harry quickly leaves the scene. In the past, anger has caused him to make magic happen by accident and he does not want to let it happen again.
Harry puts up with Marge for the rest of the week. Then her final evening comes. As they have dinner and Marge gets drunk, it looks like Harry might have just succeeded in putting up with her. Then Marge again insults Harry's parents. Harry loses his temper. Marge starts swelling oddly. The Dursleys watch in horror as Marge inflates like a balloon, until it lifts her off the floor. Harry grabs his possessions and makes a run for it.