In Brooklyn, Perry Townsend III [Richard Greene] uses his admiral-connected clout to slip into the Navy for World War I, bypassing any training whatsoever, and is immediately assigned as chief engineer of the sub chaser S. C. 599, a unit of the so-called “splinter fleet.” On the way to the ship, he fixes a stalled truck engine for Susan Leeds [Nancy Kelly], who remains unimpressed, just as is his girlfriend Anne [Joan Valerie]. Later, Townsend hitches a ride with Leeds and grows closer to her, learning that she works aboard her father’s freighter, the Maria Ann, which ferries war supplies.
A yachtsman by nature, Townsend is skeptical of his new ship and its disgruntled, slovenly crew. Yet the sailors warm to him when he treats them to a night out at a dive, paying for drinks. There, he encounters Susan again with her father’s first mate McAlliston [John Carradine], and Townsend escorts her to a stylish spot for drinks and dancing. After he sees her home at 4 a.m., she kisses him before she goes to her cabin. But her father, Captain Leeds [George Bancroft], orders Townsend to stop seeing his daughter. Townsend continues visiting anyway, and Captain Leeds eventually catches wind of the affair and punches Townsend.
Meanwhile, Lt. (jg.) John C. Drake [Preston Foster] has been found guilty of “gross negligence” for being on watch when his destroyer ran aground. He is given command of the 599, and a sympathetic superior entrusts him with four veteran seamen, including Drake’s old friend Sails Quincannon [J. Farrell MacDonald]. Drake quickly imposes discipline, while Townsend complains to the admiral and is told to do his job.
Drake is ordered to escort a convoy to Italy, which includes Susan’s freighter. On the night of Friday the 13th, lookout Pratt [Elisha Cook Jr.] raises a false alarm, followed by a genuine sighting of a periscope. The 599 sinks a German U-boat with depth charges and reaches the port of Brindisi.
Townsend, unable to speak to Susan, sends a message asking her to meet him at the Hotel Rex and tells her that he loves her. There, he proposes; she agrees, but her father arrives, punches Townsend again, and drags Susan back to the ship. Before Townsend can act, he learns that the 599 has immediate orders to sail for a strike on a submarine nicknamed “Old Man 26,” which has just claimed its 31st victim. Its Adriatic base is concealed behind a dense minefield, so Drake is only allowed to take volunteers — and every crewman volunteers. In the meantime, Captain Leeds discovers that Townsend has, in fact, arranged to marry Susan. He hurries to the 599, but Townsend knock him out before he can intervene. When Captain Leeds comes to, he learns of the mission and joins Townsend in the engine room.
Drake sinks the enemy submarine and returns in triumph. The wedding plans proceed only to be put on hold as the 599 is ordered to Malta for repairs. The Maria Ann comes alongside again, and Susan declares that she will wait for Townsend.
