When the Nazis overrun Holland, they seize its largest shipyard and turn it into a production centre for their U-Boats. Jaap Van Leyden (Ralph Richardson), the shipyard's owner, is called to a meeting with the head of the Gestapo and offered a deal; collaborate and help to boost U-Boat production - or else. But there's something the Gestapo don't know: Van Leyden is actually a hero of the local resistance and he sets in motion a desperate plan to strike back against the U-Boat production plant. There are two completed U-Boats in his yards. One becomes the target of a daring theft while the other gives Van Leyden an opportunity to launch a do-or-die assassination attempt against his unsuspecting Nazi masters.