This children's story is about a boy, Kay Harker, who sets out to discover what became of a fortune stolen from his seafaring great grandfather Aston Tirrold Harker (in reality, Aston Tirrold is a village in Oxfordshire). The treasure is also sought by a coven of witches who are also seeking it for their own ends. Kay's governess Sylvia Daisy Pouncer is a member of the coven. The witches are led or guided by the wizard Abner Brown.
Kay Harker is aided in his quest by various talking animals, most notably Nibbins the cat, who used to be a witch's cat but has reformed. There are two other household cats: the main antagonist is Blackmalkin, and he is aided by the mysterious Greymalkin who takes his name from the witch's familiar in the opening scene of Shakespeare's Macbeth.
Kay Harker experiences various adventures-—sailing on the high seas, swimming with mermaids, flying on broomsticks. At one point in the novel he manages to see into the past. Many maternal characters appear in the book, one takes Kay on a nocturnal ride on a magical horse. She then reappears at the end of the book as Caroline Louisa, Kay's new governess. She tells Kay, "..I loved your mother...". It is possible that she represents Masefield's memory of his own mother who died when he was very young.