Ten years have passed since Harry was left on the Dursleys' doorstep. Harry now lives in a small cupboard under the stairs at 4 Privet Drive. His aunt Petunia, uncle Vernon, and cousin Dudley Dursley treat him very badly. Harry has to do many chores, wears Dudley's old clothes, and is often blamed for things he did not do.
The chapter begins on Dudley's eleventh birthday. Dudley receives a huge number of presents, but he complains because he wanted even more. The Dursleys plan a special day for him, including a visit to the zoo.
A problem arises when Mrs. Figg, the elderly neighbor who usually looks after Harry, breaks her leg and cannot babysit him. Since they have no other choice, the Dursleys reluctantly take Harry with them to the zoo.
At the zoo, Dudley enjoys looking at the animals, but Harry is especially interested in a large boa constrictor. To Harry's surprise, the snake seems to understand him. Harry quietly talks to it, and the snake appears to respond.
When Dudley pushes Harry aside to get a better view, something strange happens. The glass wall of the snake enclosure suddenly disappears without anyone touching it. The boa constrictor escapes and slithers away. As it leaves, it seems to thank Harry before disappearing.
Everyone is shocked and frightened. The glass mysteriously reappears after the snake escapes. Uncle Vernon believes Harry caused the incident, even though Harry has no idea how it happened.
When they return home, Harry is punished by being locked in his cupboard. As he thinks about the strange event, he realizes that unusual things have happened around him many times before. Whenever he is in trouble, mysterious events seem to occur, though he has never understood why.
The chapter ends with Harry still unaware that he is a wizard. However, the strange events around him hint that he is different from ordinary children, setting the stage for the magical discoveries that will soon change his life.