In 1918, at the Allied Naval Base in Taranto, Italy, Lieutenant Commander T. J. Toler takes over command of an American submarine from Lieutenant Commander Thomas Knowlton. Immediately after assuming command, Toler imposes his tough and exacting personality on his sailors and orders his officers, including Knowlton and his best friend, Lieutenant Commander "Brick" Walters, to attend an officer's dance while on shore leave. Bored by the dance, Knowlton and Walters are about to sneak away when Knowlton spots pretty young Joan Standish, Toler's daughter, in a crowd of fat matrons. After the two officers compete for Joan's attention, Knowlton finally gets her alone and, while startled to discover that she is married, convinces her to accompany him to a local carnival. During the festivities, the town is attacked by German bombers, and the couple flees to safety in Knowlton's apartment. There Knowlton confesses to Joan that he loves her, but she resists his advances and is relieved when he is called suddenly to his submarine. While at sea, the submarine sinks two German battleships, and Toler orders several men, including Walters, to rescue the German survivors in a dinghy. Before the dinghy reaches the ship, however, German airplanes attack the exposed submarine, forcing Toler to dive without rescuing Walters. Although damaged by the bombing, the submarine survives and returns to the base for another shore leave. While seamen like MacDougal and "Ptomaine," the ship's cook and an aspiring dentist, cavort with boxing kangaroos and fiesty British sailors, Knowlton finds Joan working in a military hospital. There Joan introduces Knowlton to her husband Herbert, a British flight commander who was paralyzed in an airplane crash. Stunned by the introduction, Knowlton rushes off but is followed to his apartment by Joan, who finally confesses her love. After the couple pledges to remain together in spite of Herbert, Toler shows up and subtly warns Knowlton to stay away from his daughter. During the next sea patrol, Toler confronts Knowlton about the affair and forcefully advises him to terminate the romance. Later, as the submarine approaches a small fleet of German battleships, Knowlton spots Walter's dinghy in the periscope and asks that a rescue be attempted. When Toler refuses his request and orders his men only to watch the battleships, Knowlton countermands his superior and begins to bomb the Germans with torpedoes. Although two battleships are sunk, two others survive and immediately attack the submarine. The submarine is badly damaged and is forced to submerge to a dangerous depth. Although confined to the brig, Knowlton rushes to the control room when he discovers a chlorine gas leak. As the deadly gas seeps through the submarine, the sailors crowd into the control room and, using water-soaked cloth to delay the gas's effects, work frantically to start up the damaged motor. With only moments to spare, the engine is started by the surviving men, and the submarine maneuvers to safety. On shore, Knowlton is dishonorably discharged, but Joan is undeterred by the scandal and insists that Herbert be told the truth. When Knowlton learns that his rival has just been operated on and probably will walk again, however, he leaves the hospital without telling Herbert about the affair. Knowlton then returns to Joan and, in front of Toler, pretends to be drunk and callous. Shocked by Knowlton's behavior, Joan, who is unaware of Herbert's successful operation, dismisses him and prepares to return to her marriage. Before Toler's submarine leaves on a dangerous mission, Knowlton slips on board and, after revealing his sacrifice to Toler, is allowed to rejoin the crew. During the fierce battle, the submarine is hit while attacking a German fort. As the submarine sinks, Knowlton throws Toler overboard and then, while spraying the enemy with a final round of bullets, goes down with the ship.