Harry is having the best summer ever in Diagon Alley. He's free to do whatever he likes and he spends his days wandering around, working on his homework, and eating free ice cream sundaes (being the defeater of Voldemort really pays off).
One day, Harry sees the coolest thing ever: a brand new broom called the Firebolt. It's super expensive, though, so Harry restrains himself from buying one.
Harry runs errands and starts buying his supplies for the school year.
At the bookstore, he picks up his books for a new subject, Divination, which is about predicting the future. He also learns that the Monster Book he got is a course book for Care of Magical Creatures.
The books are all eating each other in a display case, much to the store owner's horror.
Harry sees the large black dog on a book cover and gets nervous – turns out the black dog is a death omen.
Finally, Hermione, Ron, and the other Weasleys arrive in Diagon Alley.
Harry goes shopping with his friends and they all head to a magical pet shop so Hermione can get an owl.
Ron also takes Scabbers to get him checked out. Scabbers has been looking sick since the Weasleys returned from Egypt.
The trip to the shop is eventful since a giant orange cat jumps on Ron.
The boys wait outside and Hermione appears with her new pet: that ugly orange cat, whose name is Crookshanks.
The trio go back to The Leaky Cauldron and have an entertaining dinner with the Weasleys.
Fred and George ham it up and make fun of Percy, who acts more smug than usual since he was made Head Boy at Hogwarts.
Mr. Weasley tells the kids that they'll be taking Ministry cars to the train station tomorrow to catch the Hogwarts Express. Everyone finds this a bit odd.
Later that night, Harry walks in on a fight between Ron and Percy. Percy can't find his Head Boy badge and Ron has lost his rat tonic.
When Harry goes downstairs to get the tonic, he overhears Molly and Arthur Weasley discussing Sirius Black: "Always the same words: 'He's at Hogwarts [...] he's at Hogwarts.' Black is deranged, Molly, and he wants Harry dead" (4.3.34).
Turns out that Black is after Harry and Arthur wants to tell him; Molly disagrees.
Arthur tells Molly that some guards from Azkaban are coming to guard Hogwarts until Black is caught and Dumbledore isn't thrilled about it.
The couple goes off to bed and a stunned Harry returns the rat tonic to Ron.
He also encounters the twins, who reveal that they stole Percy's badge and charmed it to read "Bighead Boy" (4.3.46). Good one.
Harry goes to bed and realizes that he isn't all that afraid. He's faced Voldemort, after all, and he trusts Dumbledore to keep him safe at Hogwarts.
What really upsets Harry is that with Black on the loose, he probably can't go to Hogsmeade with his friends.
Well, that's one way to deal with learning that a homicidal maniac is after you.