 Lt. Ward Stewart (Tyrone Power) aboard the U.S. submarine "Albacore" spends his off-duty time recalling his meetings with Jean Hewlitt (Anne Baxter)at the sub base in New London, Conn. He and a boarding party, investigating a small boat that turns out to be a Nazi Q-Boat, barely escape the ambush as sub Commander Dewey Connors (Dana Andrews) cooly keeps the sub surfaced until the men are back aboard safely. The sub returns to New London, where Ward looks up Jean, and discovers she is Connor s fiancee. Tension between the two men is high when the sub is ordered back to sea on a secret mission. Trailing a suspicious-looking tanker in the south Atlantic, the sub follows the ship through a heavily-mined channel to a small island, which is a Nazi supply base. Ward offers to lead a party ashore and destroy the base. He and his commandos accomplish their mission but only at the cost of the life of Chief Petty Officer McDonnell (James Gleason), who stay behind, in defiance of Ward s orders, to cover their escape.