The story begins with a description of Hester, who has trouble loving her three children. Hester she feels unlucky because her family is running out of money, but she cares a great deal about appearing to be wealthy. The house seems to constantly whisper, “There must be more money!” and Paul (Hester’s young son) in particular becomes concerned about the family’s financial situation. When he asks his mother why they don’t have enough money, she explains to him they they are unlucky, and that luck is the reason people are rich. Paul claims that he is lucky, but his mother doesn’t believe him, so he becomes determined to prove his luck to her.
Paul obsessively and furiously starts riding his rocking-horse because he believes it can take him to luck—a habit he keeps secret from everyone else. He also talks with Bassett, the family’s gardener, about horse racing and places bets on the races whenever he “knows” who will win. Paul’s Uncle Oscar finds out about Paul’s betting and begins betting based on Paul’s recommendations, which are always correct. Paul makes an extraordinarily large amount of money, but he also becomes increasingly anxious and intense.
Uncle Oscar helps Paul give some money to his mother anonymously, but the money only makes the whispering in the house worse. Instead of using it to pay off debts, Hester buys new furniture and invests in sending Paul to an elite school. Paul is more determined than ever to make the whispering stop, and he refuses to stop riding his rocking-horse, even when his mother suggests that he is too old for the toy. The Derby (a big horse race) is coming up, and Paul is obsessed with picking the winner.
One night, while at a party, Hester is overwhelmed with anxiety about Paul. She calls the nurse to see how he’s doing, but when the nurse offers to check on him in his room, Hester decides not to bother him until she gets home. When she finally arrives at his room, she hears a familiar yet violent noise coming from behind the door. Paul is riding his rocking-horse so hard that he and the horse are lit up in a strange light. He announces in a deep voice, “It’s Malabar” and then collapses to the floor.
Days later, Paul is very ill. Bassett tells Paul that Malabar (a horse’s name) won the Derby, and Paul now has eighty thousand pounds. Paul is very excited to be able to prove to his mother that he is, in fact, lucky. But that night, Paul dies. Uncle Oscar suggests that Hester is better off having eighty thousand pounds instead of a strange son—but that Paul is also better off dead than living in a state where “he rides his rocking-horse to find the winner.”