Against his personal preference, PT boat commander Ward Stewart is made executive officer of the submarine USS Corsair. On leave before sailing, he meets schoolteacher Jean Hewlett and gives her a romantic rush...unaware that she's the sweetheart of Dewey Connors, his new commander. At sea, the men bond while fighting German Q-ships. When will they discover their mutual romantic rivalry? Will it interfere with a spectacular commando raid on a secret German base?

Lt. Ward Stewart delights in the maneuverability and speed of his PT boat, and is disappointed when his uncle, Admiral Bob Stewart, asks him to join the submarine branch of the Navy. Stewart agrees that it is his duty to serve where he is most needed, but requests a weekend leave before assuming his new post as executive officer aboard the Corsair , which is captained by Dewey Connors. During his train trip to Washington, D.C., Stewart meets teacher Jean Hewlitt. Stewart is instantly attracted to Jean, but his rakish behavior irritates her. He persuades her to go to a party with him, but she does not keep their dinner date for the following evening. Undeterred, Stewart is pleased to discover that she teaches at the Bromley School for Girls, located in New London, Connecticut, which is also the site of the Naval submarine base. Unknown to Stewart, Jean is Connors' girl friend, and after her return to New London, Jean, who wants to believe that she prefers the stable Connors to the devil-may-care Stewart, asks Connors if they can marry before he leaves on his next mission. Connors wants to wait until after he receives a promotion, however, so he tells Jean that they can marry when he returns. The Corsair sails that night, and during the journey, black messman Oliver Cromwell Jones discovers that his friend McDonnell is secretly taking nitroglycerin pills for a heart condition. One afternoon, the sub is fired upon by a German Q-boat disguised as a Swedish scow, and Connors decides to play "possum" and allow the Germans to think that they have been sunk. The strategy pays off and the Corsair sinks the Q-boat, then returns to New London. After their arrival, Connors goes to Washington, where he receives a promotion, while Stewart continues his pursuit of Jean, who does not know that he serves under Connors. Meanwhile, McDonnell explains to Oliver that he needs to go on one more mission to atone for his cowardice during World War I, and Oliver promises to keep McDonnell's condition a secret. One evening, Stewart takes Jean to meet his feisty grandmother, then proposes to her the next morning on their way back to the school. Deeply in love with Stewart, Jean accepts, but when Stewart learns soon after that Connors intends to propose to her as well, he informs Jean that they can no longer see each other. As Jean is telling Stewart that he is the one she loves, they realize that Connors has overheard their conversation. Before the situation can be resolved, the men are sent on a mission to find a secret German naval base. Stewart tries to explain matters to Connors and maintain their friendship, but Connors coldly repulses him and attends only to business. Just as the Corsair is getting low on fuel, they find the base and follow a tanker into the mine-strewn harbor. Stewart leads the landing party, which blows up the ammunition dumps as the Corsair fires on the anchored ships. McDonnell is killed in the raid while saving Oliver and Stewart, who swim through the oil-covered, flaming water back to the sub. Because the periscope has been damaged, the fleeing sub cannot resubmerge, and Connor is forced to stay on deck and guide her through the mines and bombardments from land and sea. He is wounded but not fatally, and Stewart gets him below just before the Corsair submerges. The men make peace, and soon after the Corsair 's return to New London, Stewart and Jean are married.