It is the last watch of the lightship that stands at anchor in the Baltic Sea, near the port of Kiel; it is nine years since the end of World War II and the mines she warned the shipping of have been removed. But on her last watch, her peace is interrupted: her captain, Freytag, takes in three men whose boat has broken down, and with them, their illegal cargo of guns.
Held hostage on the ship by the three criminals, headed by the sinister Dr Caspary, the tensions between captain and crew become apparent. Freytag still hopes for a peaceful end to the watch; but his son, Fred, and his resourceful but outgunned crew cannot forget Freytag's dishonour in the war. As a stalemate develops, Freytag finds he must make a final choice - one that must redeem him in the eyes of his men.