This fast-paced comedy-thriller stars Leslie Banks as an inventor during the Second World War working on a new bomb sight at his home in a remote Scottish glen. He is unaware that he is the target of a Fifth Column organisation operating out of Glasgow who have already installed an agent in his household masquerading as the cook. Also in the house are Banks' wife Jeanne De Casalis, daughter Carla Lehmann and assistant Michael Wilding, plus a Scotland Yard detective, disguised as their butler, who suspects Wilding of passing on Banks' secrets.