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.
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.