Worried by the activities of a gang of smugglers of dope and of cosmopolitan crooks, Scotland Yard enlists 'Steeley' Delaroy and 'Tubby' Wilde as special investigators.

Their search leads to Castle Deeping, an old moated ruin in Hampshire and the scene of a recent, and unsolved murder mystery, where they are ambushed in the cellars. Steeley and Tubby escape from a watery grave when the crooks flood the moat but not before the identity of the leader is discovered.

Steeley's plan to get accepted into the gang by posing as a crook just released from prison is successful, but an exciting fight ensues when he and Tubby are recognised by the Boss.