When Harry arrives downstairs for breakfast, nobody blinks an eye. Uncle Vernon is buried behind the morning paper, and his son Dudley is sulking furiously about the grapefruit that Aunt Petunia is slicing into quarters. Dudley's school has written to the Dursleys, saying that they have no knickerbockers to accommodate his gigantic behind. Dursely has gone on a diet. Aunt Petunia has placed the whole family under Dudley's diet, which Harry has survived by asking his friends to send him food by owl.
Breakfast is interrupted by the arrival of the postman with a letter, and Uncle Vernon calls Harry into a side room. He is furious that the letter is from Ron's mother, Molly Weasley, who is inviting Harry to attend the Quidditch World Cup and stay with her family for the rest of the summer. What most infuriates Uncle Vernon is that the letter is in an envelope covered in dozens of stamps. Uncle Vernon makes it clear that he doesn't want Harry around for the rest of the summer, but that he doesn't want Harry to get his way by getting to leave. Finally Harry mentions casually that he must finish a letter to Sirius Black, and Uncle Vernon looks terrified and grumpily tells Harry to go, not wanting a convicted murderer to show up thinking that Harry is being mistreated.
Harry bounces upstairs delightedly to find that his snowy owl, Hedwig, has returned from a night flight, and that another, smaller owl is zooming around the room. The smaller owl, Pig, is Ron Weasley's new owl, and it delivered a note from Ron saying for Harry to be ready to leave for the Quidditch Cup the following night at five. Harry writes back to Ron and sends his letter out to Sirius, feeling most optimistic about the end of the summer.
Harry packs his trunks and is ready to leave by noon the next day; the Dursleys are all silent and terrified. Uncle Vernon makes several snide comments about how he hopes that the Weasleys will think to dress properly. Five o'clock comes and the Weasleys have not yet arrived. Finally, at a quarter past five, loud banging sounds come from inside the Dursleys' boarded-up fireplace, and Harry knows instantly that the Weasleys have tried to travel by Floo Powder through the fireplace network. Arthur Weasley's voice warns Harry to stand back, before the fireplace explodes, revealing the red-haired figures of Ron, the twins Fred and George, and their father, Arthur. They greet the Dursleys politely, but the Dursleys simply stare, while Dudley disappears, clutching his bottom, which spouted a pig's tail during his last encounter with a grown wizard.
Fred and George head upstairs to retrieve Harry's trunk from his room, while Mr. Weasley bravely attempts conversation with the Dursleys, who are unresponsive. When the twins return with Harry's trunk, one of them drops a pocketful of sweets on his way back to the fireplace. Harry and Mr. Weasley are the only two wizards remaining in the living room. They find that Dudley has devoured one of the sweets, thus enlarging his tongue. Aunt Petunia shrieks and tries to pull his tongue out of his mouth, while Uncle Vernon throws ornaments at Mr. Weasley. Mr. Weasley tries to restore the tongue.
These chapters contrast the real world that we recognize and the magical wizard world. To reach Hogwarts, one only must walk casually between two train platforms; here, we also see the boundary between those worlds, and the ease with which Harry steps into the fireplace and crosses it. This makes the Wizard world familiar and imaginable. J.K. Rowling has said that one of her goals was to create a world accessible to those who know where to look. Furthermore, Harry's craftiness in getting permission to join the Weasleys is as new step in Harry's ability to overcome the boundaries imposed on his life by his relatives. With each book, his escape into the world of Magic requires Harry to take more initiative. In this fourth book, Harry is bolder, cleverer, and more experienced in escape tactics.
The introduction of the two families portrays each character humorously. The kind and curious Mr. Weasley does everything he can to make conversation with the Dursleys, who stare at him in frightened and confused dismay. The Weasley twins are polite and helpful, but their manners are laced with mischief. Harry watches everything with amusement and fascination, as does Ron. This face-off between extreme representations of the two respective worlds provides a transition that reflects Harry's shift from one to the next.