Harry is not in the best shape when the novel begins—he feels isolated among the unbearable Muggles, misses Hogwarts, and feels abandoned by his two best friends, Ron and Hermione—and his unhappiness leads him to act recklessly. Given Harry’s close and terrifying encounter with Lord Voldemort at the end of Book IV, Harry is right to be intensely concerned about the fate of his beloved Hogwarts, and the total lack of news from the Wizard world is excruciating for him. Harry’s prolonged aggravation culminates in the alleyway with Dudley, when he whips out his wand and comes terribly close to using his magic to torment him. Harry is barely able to contain his rage, and, were it not for the interruption of the dementors, he very well might have cast a spell on Dudley, earning himself instant expulsion from Hogwarts. In this sense, the dementors actually save Harry from his own evil desires.
Though the Wizard and Muggle worlds operate under very different sets of principles, these principles sometimes overlap, and the sharp delineation between those worlds is beginning to blur. Despite his concerns about Hogwarts and his Wizard pals, Harry hides in the flowerbed in order to hear news from the Muggle world. He is already concerned that Voldemort’s effort to regain power could penetrate the Muggle universe, having a dramatic and debilitating effect on everyday Muggle life. Aunt Petunia’s recognition of Voldemort and her obvious fear at the sound of his name, which mirrors the typical Wizard reaction, indicate that Voldemort’s evil may have already found its way into her life. Harry’s worlds are getting all mixed up, with dementors showing up in Surrey, Mrs. Figg turning out to be a Squib, Aunt Petunia knowing about the prison at Azkaban, and Uncle Vernon asking questions about the Ministry of Magic. The principles Harry thought were specific to each of his worlds turn out to cross over to the other world with unexpected ease.
Rowling uses these opening chapters to introduce a series of questions that she will proceed to answer in the thirty-six chapters that follow. We don’t yet know why Aunt Petunia received a Howler, presumably from Dumbledore, or why she knows about Azkaban and Lord Voldemort. Why haven’t Ron and Hermione been writing? Why hasn’t Harry heard from Dumbledore? What’s going on with Voldemort? Why were dementors sent to attack Harry in Little Whinging? Of all these questions, the last one is the most pressing, since the presence of the dementors in Little Whinging is curious on many levels. The dementors should never present themselves to Muggles, and, more important, they should never abandon their post guarding the prison at Azkaban, where they work under the direction and control of Cornelius Fudge and the Ministry of Magic. The fact that the dementors acted so fully out of character does not bode well for Harry and his friends.