One Saturday morning, Harry tries out a spell scribbled on the margins of his favorite textbook. Immediately, a sleeping Ron across the room is pulled out of his bed and levitates above it, hanging upside down.
In the morning, Ron tells Hermione the story of what Harry did to him, which he thinks is hysterical. Hermione, however, doesn't approve.
Harry and Hermione bicker about Harry's use of the Half-Blood Prince's textbook. She is not only worried about the ethics of using someone else's book, but she is also worried about the danger of the spells and information inside. She says that the Prince's spells might not be Ministry-approved, and that the Prince seems like a potentially dangerous guy.
Later, everyone goes on an outing to Hogsmeade.
While on his outing, Harry is confronted by Slughorn, who asks why Harry never comes to any of his parties. Harry blames it on Quidditch practice. Slughorn tries to lock Harry down for another one of his gatherings, but Harry tells him he has an appointment with Dumbledore.
Harry sees Mundungus Fletcher outside of the Three Broomsticks tavern.
Fletcher drops a suitcase he is carrying, and the contents spill out.
Harry realizes that Fletcher has been pilfering items from Sirius Black's house (now Harry's house) at Grimmauld Place.
This realization sends Harry into a fury and he attacks Fletcher, but Fletcher gets away before Harry can do anything.
Once inside the tavern, Ron and Hermione try to calm Harry down with some butterbeer, but Harry is thoroughly shaken at the thought that someone might be stealing and selling his dead godfather's possessions.
Finally, the three decide that they've had enough.
They leave the Three Broomsticks and brave the dismal weather, following close behind Katie Bell and her friend.
Katie Bell and her friend get into an argument as they are walking. Katie is holding a package. The package falls to the ground and Katie is immediately levitated into the sky, and she is writhing in pain and agony.
Below her there is an opal necklace peeping through the packaging.
Harry runs to get help, which comes in the form of Hagrid.
Hagrid rushes Katie to Hogwarts for medical attention.
It would seem someone at the Three Broomsticks someone used an Imperius curse on Katie (to control her). Her friend tells Harry that Katie looked all funny when she came out of the bathroom at the tavern, and that she was carrying a mysterious package, which she was supposed to deliver to someone in Hogwarts. When the package tore open, Katie must have touched the cursed necklace inside it.