The Five are sent away to Smuggler's Top, the home of Mr Lenoir, a fellow-scientist of Uncle Quentin, after a tree blows onto Kirrin Cottage. Smuggler's Top is a weird house in a town surrounded by dangerous marsh. They meet Pierre "Sooty" Lenoir (Julian's and Dick's schoolmate and the son of Mr Lenoir) and his half-sister Marybelle. Mr Lenoir hates dogs, so Timmy has to spend his time there hidden in secret passages. The boys spotted a light flashing in the house's tower. They think it is Block (the sinister manservant) in the tower but then see him in bed at the same time. Uncle Quentin comes to stay with them but mysteriously disappears, along with Sooty. The Five find yet another secret underground passage leading to where Sooty and Uncle Quentin are being held captive with Timmy. They fight off the kidnappers and go back to the house, where they realise Uncle Quentin and Mr Lenoir are planning to drain the marshes, stopping the smugglers, and Block has been put into their household to stop the plans. They call the police to capture the smugglers, using Timmy to find them in the underground tunnels.
