Mainwaring fears the Nazis may send spies across the Channel, so he orders the lighthouse manned by Jones, Pike, Godfrey and Frazer, who tells a terrifying story. The trio accidentally start the mechanism and are unable to shut it down, their attempts only make it worse until the mighty headlight points to town and the siren blows. On shore, the ARP goes berserk, especially when a Luftwaffe attack approaches, while Mainwaring, Wilson and Walker desperately try to find out how to get to their party without a boat or telephone line.