The film begins with five children (Robert, Cyril, Jane, Anthea and The Lamb), whose father has gone to fight in World War I. Consequently, the children must meanwhile stay at their aunt and eccentric uncle's house with his unpleasant son, Horace. While exploring the house, Robert finds a locked door in the forbidden greenhouse and brings the other children. They manage to open the door, which leads them through a secret path to the beach surrounding the house. There they discover a large shelled creature, which reveals itself as a "psammead crustacean decapodlium wishasaurus," or sand fairy for short. The children, befuddled by this confusing name, refer to the creature simply as "It." It seems rather mischievous and Cyril doubts whether the children should trust him, but upon confirming that It can grant wishes, the children wish for all the house chores on their list to be done by magic. When they return to the house, they see dozens of copies of themselves doing the chores and wrecking the house in the attempt.