An Allied diving team fights off the Nazis while trying to salvage a downed enemy ship.

In 1943, a British naval officer from Canada, Lieut.-Comdr. John Bolton, loses his submarine and most of his crew in a raid on the "invincible" German battleship Lindendorf. Although Bolton is cleared of charges in a court of inquiry, the few survivors from the encounter hold him responsible for the deaths of their shipmates. A short time later, Bolton is assigned by Vice-Admiral Redmayne to train three 4-man crews along the northern coast of Scotland for a trio of midget submarines equipped with side cargoes of amatol explosives. Because the exact nature of the mission is kept secret, and because four of the men are survivors from the previous expedition, Bolton's rigid disciplinary measures are met with open hostility. However, the men unite when a German reconnaissance plane is sighted over the camouflaged training base, and German paratroopers land in the area. The Admiralty reveals that the midget subs are to attack the Lindendorf while she is undergoing repairs inside a Norwegian fjord. The subs make their way through heavily mined waters to reach the fjord, and crew members battle German frogmen underwater. One of the subs, the X-2, is sunk by a German E-boat's depth charges, and a second, the X-1, is scuttled by its crew before they are captured and taken aboard the Lindendorf. Before the men from the X-1 can be subjected to forced interrogation, however, the X-3 fires its explosives and destroys the Lindendorf.