Siblings Julian, Dick and Anne are spending the Easter school holidays with their cousin George at her parents’ house, Kirrin Cottage.

After a tree falls on the house, the four children are sent to Smuggler's Top, the home of Mr Lenoir, a fellow-scientist of George's father, Uncle Quentin.

They suspect the signaller may be Block, the house's sinister, apparently deaf manservant, but he appears to be sleeping in his bedroom at the time.

However, Uncle Quentin is kidnapped, along with Sooty, by a local smuggler named Mr Barling, who fears the planned draining of the marsh will bring an end to his smuggling activity.

The children find yet another secret underground passage, leading to where Sooty and Uncle Quentin are being held captive by Barling and his henchman, who have been assisted by Block.