Four New York City siblings decide to pool their resources so that each can do a special thing on the Saturday that is his turn to receive the combined allowance.
The Melendy family moves to a house in the country where a secret room, a cupola, a stable, and a brook provide Mona, Rush, Randy, and Oliver with adventures far different from the city life to which they are accustomed.
Left alone when Rush, Mark, and Mona go away to boarding school, Randy and Oliver are lonely and bored until a mysterious letter brings the first of many clues to a mystery that takes all winter to solve.
