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. Smuggler's Top is a queer house at the summit of an old hilltop coastal town, partly surrounded by dangerous marsh. The children are welcomed by Pierre "Sooty" Lenoir (Julian's and Dick's schoolmate and Mr Lenoir's stepson) and his half-sister Marybelle. Mr Lenoir hates dogs, so George's dog Timmy is hidden in a secret passage. One night, the boys spot a light flashing in the house's tower. 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. Uncle Quentin comes to stay with them and intends to sell Mr Lenoir a plan for draining the marsh. More excitement follows.