Lt. Lionel Crabb, RNVR, is sent to the Royal Navy base at Gibraltar as Bomb and Mine Disposal Officer. Rather than simply dispose of unexploded mines and explosives he wants to locate enemy devices and protect the ships anchored in the harbour. He assembles a team of divers. Their greatest enemy is Italian frogmen riding "chariots" - human torpedoes. Recently two British battleships were severely damaged in Alexandria harbour and now they've shifted their attention to Gibraltar.—grantss