A certain "skinny, black-haired, bespectacled boy" (1.2) we all know and love is lying underneath the open front window of his aunt and uncle's house on Privet Drive, Little Whinging, Surrey, England.
It is summer, and the boy (yes, it's Harry Potter) is stuck with his relatives while school (i.e. Hogwarts School for Witchcraft and Wizardry) is out for vacation.
Harry is lying under this open window because his Aunt Petunia and Uncle Vernon won't let Harry listen to the news.
But Harry has to listen to the news — he's worried that his nemesis, the evil wizard Lord Voldemort, is going to start going around killing people.
If Voldemort murders large groups of people, surely even Muggle (non-wizard) news would carry the story?
But there's nothing unusual on the news.
So, Harry has no idea what is going on with the war against Voldemort.
Harry hears a loud crack from the road and draws his wand for self defense.
Uncle Vernon grabs Harry by the throat through the open window and shouts, "Put — it — away! [...] Now! Before — anyone — sees!" (1.23).
Harry, Uncle Vernon, and Aunt Petunia all squabble over Harry's magic use, which Uncle Vernon and Aunt Petunia think is perverted and shocking.
Harry is furious: his relatives continue to be bullying and abusive, none of his friends have been writing to him, and he is totally cut off from the wizarding world.
At the end of Book Four, Harry saw Voldemort come back to life and murder one of Harry's Hogwarts classmates, Cedric Diggory.
Now, Harry is stuck by himself, with nothing to do but brood about Voldemort and what he might be doing while Harry is wasting time with his uncle and aunt.
No wonder Harry is fuming!
Harry sees his cousin Dudley with a group of his friends.
Dudley is a stupid, bullying brute.
Harry is looking for a fight, so he starts laughing at Dudley.
Dudley mocks Harry for crying out at night: "‛Don't kill Cedric! Don't kill Cedric!' Who's Cedric — your boyfriend?" (1.126).
Harry is so furious that Dudley dares to laugh about his nightmares and Cedric's death that he pulls his wand on Dudley.
Dudley suddenly gasps and cries out, "W-what are you d-doing? S-stop it!" (1.141).
But it's not Harry who is making Dudley feel cold — it's a Dementor, a magical creature that sucks the joy and life out of its victims.
Harry uses his wand to cast the Patronus spell at the Dementors, and finally, "an enormous silver stag erupted from the tip of Harry's wand; its antlers caught the Dementor in the place where the heart should have been; it was thrown backwards [... and] the Dementor swooped away, bat-like and defeated" (1.173).
Harry has no idea how a pair of Dementors got to Little Whinging — they're supposed to be acting as guards for Azkaban, the wizard prison.
Dudley is practically unconscious from excessive Dementor exposure.
Just then, Mrs. Figg, Harry's elderly neighbor, comes by and shouts something completely unexpected: "Don't put [your wand] away, idiot boy! [...] What if there are more of them around? Oh, I'm going to kill Mundungus Fletcher!" (1.181).