It is midnight when the story opens. Harry is lying quietly on his bed, trying to write an essay about witch-burning for his Hogwarts classes without waking up his aunt, uncle, and cousin, all of whom would be horrified to know that he was engaged in any form of magic while in their house. Harry realizes that it is now one o'clock, an hour past his thirteenth birthday. Since the Dursleys have never celebrated his birthday, Harry does not consider this event remarkable. Harry reminisces about his parents' death at the hands of Lord Voldemort, and on his own encounter with Voldemort the previous spring at Hogwarts. Harry feels relieved to have lived to see his thirteenth birthday.
Harry notices an odd flapping thing coming toward his window, and he soon sees that it is three owls, his own loyal, snowy Hedwig, a second unfamiliar owl, and held up by the first two, Errol, his friend Ron Weasley's feeble family owl. The owls deliver birthday cards and gifts from Ron, Hermione, and Hagrid, his three closest friends. From Ron, Harry receives a pocket Sneakoscope and a letter describing a family trip to Egypt. From Hermione, he receives a Broomstick Servicing Kit (which Harry greatly appreciates, since his favorite sport, Quidditch, is played on broomstick). From Hagrid, he receives a schoolbook entitled "The Monster Book of Monsters." Lastly, Harry receives a letter from Hogwarts requesting forms from guardians allowing the students to venture into the wizard village of Hogsmeade; Harry knows that he will not be able to persuade his uncle or aunt to sign the form. Nevertheless, Harry glances again at his cards and falls asleep that night feeling, for the first time ever, that his birthday was a special occasion.
Harry walks down to breakfast the following morning to find his cousin Dudley, his uncle Vernon, and his aunt Petunia watching a television report about a dangerous escaped convict named Black. The Dursleys ignore Harry, as usual, and Vernon leaves to pick up his sister, Aunt Marge, at the train station, first warning Harry to pretend that he is "normal." Harry is horrified at the news, as Aunt Marge does nothing during her visits but cuddle her dangerous dogs and bark insults at Harry. Harry suddenly has an idea, and he bargains with Vernon that he will act normal and pretend he attends school at a center for incurably criminal boys, if afterwards Vernon will sign his Hogsmeade form. Vernon is not pleased, but he agrees.
Aunt Marge arrives, roaring praise at Dudley, sharing her tea with her dog, Ripper, and asking Harry whether he is being caned enough at his school. Harry answers the questions as carefully and politely as he can, but grows increasingly angry as Aunt Marge criticizes Harry's parents, saying of his mother, "If there is something wrong with the bitch, there'll be something wrong with the pup." Harry's anger causes her wine glass to break, and from that point on, Harry is even more cautious. Finally, one night at dinner Aunt Marge says flippantly that Harry's parents were irresponsible enough to get themselves killed in a car crash, and Harry is so livid that he causes her to expand and float. Chaos ensues, and in the middle of it Harry collects his things and leaves, threatening Uncle Vernon with his wand when his uncle tries to stop him.
The beginning of each Harry Potter book shows Harry in a miserable, abusive situation while living with the Dursleys, and then traces his method of escape from the stifling world of these particular Muggles—the wizards' term for non-magical people—over the threshold into the wizard world. This book acknowledges both the "ordinary" and the "magical" worlds, and it allows continuity between them, enabling us to experience the brilliance and novelty of Hogwarts with Harry.
Harry's method of escape shows his development as a character throughout the series. In these chapters, Harry takes more initiative than ever before in escaping his summertime plight and returning to Hogwarts; in the first book, Hagrid rescues him from the Dursleys; in the second book, Ron comes to his window in a flying car. Here, Harry sets himself free, thus signifying a step in his further confidence in his role in the magical world, in addition to his own maturation as an adolescent. Harry has reached a point where he understands his own personal thresholds, and he now has the boldness to act when these thresholds are crossed, such as when Aunt Marge insults his parents. Harry takes initiative not only to sneak his magical homework into his room, or to bargain with Uncle Vernon on signing the Hogsmeade form, but furthermore, to grab his trunk, brandish his wand at his uncle, and leave the house when he has had enough. Of all of Harry's journeys into the magical world, this one requires the most independence and courage.
Harry's friends' gifts each reflect their personality and foreshadow a later event in the story. Hagrid sends a biting monster book. Hagrid loves monsters dearly, and in this novel, his loyalty to one monster in particular constitutes a great part of the plot. Ron sends a sneakoscope and a picture of his family in Egypt; the same picture inspired Black to leave prison, and the sneakoscope will alert Harry and his friends when they were being overheard. Hermione sends a broomstick-servicing kit. This gift acknowledges Harry's deep devotion to his broomstick-sport, Quidditch; her thoughtfulness with this gift foreruns her later concern that his new broom was sent by Black.