With the Allies poised for the invasion of Italy, the Germans hold a vital card: a floating dock, the only one in the Adriatic large enough to take a major warship. Moored at a small port near Rimini, it is exposed, vulnerable. It must be destroyed before the Germans can tow it to a safer harbour. Air, surface and conventional submarine attacks are out of the question. Only one team can do the job and still stand a chance of surviving - the crew of HM Midget Submarine XE 51.