A submarine officer clashes with his commander while in love with his married daughter in this adaptation of Commander Edward Ellsberg's novel Pigboats.
In 1918 "Dead-pan" Toler (Walter Huston) takes command of a submarine in the Adriatic Sea and goes by the book. He gives MacDougall (Eugene Pallette) $10, because loaning is against regulations. Toler orders Tom Knowlton (Robert Montgomery) and Brick Walters (Robert Young) to attend an officers dance where they meet his daughter Joan Standish (Madge Evans). Knowlton gets close to Joan when facing death during an air raid even though she is married. He asks if she is afraid of herself and says he loves her.
Toler fires torpedoes at a German ship and sends Walters in a boat to get the codebook; but when planes attack, he has to order the sub to dive. Knowlton keeps shooting, but MacDougall hits him and takes him below. Toler tells Knowlton he must obey orders. On leave would-be dentist Smith (Jimmy Durante) fights an Australian for $5 and is pummeled by a kangaroo; finally he slugs a British marine. Knowlton visits a hospital to find Joan and meets her crippled husband. Joan consoles Knowlton for the death of Walters and says she loves him but also good-bye. They kiss; Toler comes in and toasts discipline. Joan leaves with her father. At sea Toler argues with Knowlton about Joan but blames himself for bringing it up. Knowlton sees the dingy and maybe Walters; but Toler has orders not to attack the enemy ships. Knowlton fires the torpedoes himself, sinking two of the three ships. Toler puts Knowlton under arrest. Depth charges causes the sub to leak water. Seamen Jenks (Sterling Holloway) stops a loose warhead and is injured. Toler orders the sub to the bottom. Knowlton discovers chlorine gas and closes off that section. Toler orders the sub up, but the engine won't start. Forgotten by MacDougall, Jenks is left to suffocate in order to save the rest. As oxygen gets low, one man shoots himself. Toler gets the engine started using emery stone in the clutch.
Toler volunteers to attack an enemy port by blowing up its tower in order to bottle up the subs. Knowlton under arrest is visited by Joan, who says she loves him. Knowlton is dismissed from the Navy. Toler tells Knowlton he is sharing his disgrace with Joan. Knowlton visits her husband after his operation. Then he quarrels with Joan, and Toler hits him. Knowlton stows away on the sub and tells Toler he wasn't drunk. Toler realizes that Knowlton sent Joan "back to her duty" and apologizes for hitting him. In the attack Toler orders his men to abandon ship. Knowlton pushes Toler overboard and blows up the tower with the ship.
This dramatic story not only reflects the submarine warfare of the first world war but foreshadows that of the second. The romantic first officer clashes with the disciplined commander but gradually comes to understand duty and sacrifices himself. In the larger view the hell of submarine warfare is that of destroying others or being destroyed because of the major political bungling that caused the war.