A Harvard graduate serving on a battleship is faced with the realities of war.

At a West Coast Naval base, just after America's entry into World War II, Harvard-educated Lieut. Gregg Masterman, aide to Rear Admiral Stephan "Iron Pants" Thomas, incurs the ire of up-from-the ranks officer Lieut. Comdr. Martin J. Roberts because Roberts thinks Masterman is delaying his ship's return to sea. When Roberts goes to Thomas to complain, Thomas recognizes Roberts from their World War I service and assigns him to be skipper of the "mothball" destroyer the U.S.S. Warren . That night, Masterman is chagrined to learn that Thomas has made him Roberts' executive officer. Masterman goes to see Roberts on the Warren and encounters former chief yeoman Henry Johnson, who had served on the ship during World War I, and has been her civilian caretaker. Johnson enthuses both officers about the decaying Warren when he tells them the ship's proud history, but they remain cool to each other, especially after Roberts tells Johnson he cannot serve again because the war will be "too tough for grey hairs." Roberts warns Masterman that sentiment has no place when making life and death decisions, but on his way out, Masterman suggests to Johnson that without grey hair, he would look much younger. On the first day at sea, a now dark-haired Johnson is in the ranks, having obtained Masterman's help in getting back into the Navy. Despite their differences, as the voyage gets underway, Roberts and Masterman gain respect for each other. Meanwhile, Thomas is reassigned to head a Navy convoy and summons the Warren to replace one of his damaged vessels. On the way, the Warren is attacked by a Japanese plane, which then manages to get away, in part because of a mistake Masterman and his men make. Masterman blames himself, but Roberts tells him not to be too hard on himself. A short time later, a violent storm arises, during which Johnson is knocked unconscious. When pharmacist's mate "Doc" Miller says that the pounding sea might hinder Johnson's recovery, Masterman reduces the ship's speed, against the captain's orders. Roberts reverses Masterman's decision and warns him that command means making unpopular decisions. Nearing Thomas' convey, the Warren encounters a lifeboat occupied by two women and twenty infants, survivors of a torpedoed rescue ship. One of the women is about to give birth and, as there is no doctor on board, carpenter's mate "Chips," whose wife is a nurse, helps with the delivery. As the ship reaches the convoy, sailors in the Warren 's "baby division" take their charges on deck for some sun. Looking through binoculars, from their ship, the Chattanooga , Thomas and his officers see a naked baby crawling on deck of the Warren and mistake it for a pig. Soon set straight by communiques from Roberts, Thomas congratulates them on their successful "launch" of the new baby. Now the other female on board, Mrs. Collins, goes into labor, but is having a very difficult time. Masterman wants to transfer her to the Chattanooga , because there is a doctor on board, but Roberts refuses because both ships would have to stop. Thomas, who is also worried about the newest "launch," recalls a British naval tradition from the days when women accompanied their husbands to sea, whereby difficult births were helped along by cannon fire. Just then, Japanese planes from an approaching carrier bomb the Chattanooga , destroying its rudder and making it go in circles in the fog. As the Warren joins the battle, Roberts is hit and temporarily relinquishes command to Masterman. Now that Masterman is in charge, he, too, realizes that decisions must be based on the need of the ship, not sentiment. During a fierce battle, Johnson, who has recovered, rises from his bed and helps the Warren destroy the Japanese battleship, and Mrs. Collins gives birth to a healthy baby boy. Some time later, when the convoy arrives in San Francisco, the Chattanooga is towed by the Warren , after which Roberts, Masterman and Johnson are each awarded the Navy Cross. When the Warren returns to sea, Roberts and Masterman contentedly stroll the deck together.