A surprise visit from his brother almost blows an undercover agent out of the water.

When a ne'er-do-well, ex-war correspondent Sam Gallagher, asks his brother Jeff for a job at the Calship shipyards where Jeff is a foreman, Jeff, bitter about Sam's extended absence during their mother's illness and death, reluctantly hires him as a pile driver. Unknown to Jeff, Sam is an undercover agent for Naval Intelligence and has been assigned by James Thane to investigate reports of sabotage at the shipyards. Thane tells Sam that, as a cover, a family consisting of a wife and two children has been fabricated for him, and a house has been rented in his name. Before dawn the next morning, Sam reports to work and meets his new partner, Red Kelly, a skilled veteran who detests all newcomers. After Sam is assigned the cold, wet job of driving piles into the sea floor, he meets fellow workers Brownell and Simms; Max Lessing, the plant's general manager; and Ben Royall, the safety inspector. While filing some papers at the office, Sam sees Lea Damaron, his former sweetheart, who is now dating Jeff and working as a secretary at the plant. When Sam tells Lea that he is married and has two children, Lea passes the news along to Jeff. Jeff, suspicious of his brother's behavior, determines to investigate his past. After work, Sam goes to his new house and there meets Jill McCann, the agent assigned to pose as his wife, and Joan and Paul, refugees orphaned by the war who are to act as his children. Soon after, Miller, another worker at the plant, visits and introduces himself as Sam's contact. Miller informs Sam that he has met two Nazis posing as defense workers and learned that they are planning to sabotage the entire shipyard. The next day at work, Kelly's antagonism toward Sam results in a fistfight. When Sam feigns cowardice, Jeff's suspicions are aroused and he asks Lea to get Sam to confide in her. While driving Sam home after work, Lea questions him about his wedding, and he tells her that he was married in London. When Sam invites Lea inside to meet his wife Jill, Jill mentions that they were wed in Paris, thus confirming Jeff's suspicions. At the plant that night, a saboteur stealthily removes a bolt from a set of steel beams. During a meeting the next day, Lessing announces that a dock must be extended to meet a carrier due to arrive in three days. To speed progress, the pre-constructed set of metal beams is lifted by a crane to attach to the dock, but with the bolt missing, a nut loosens and falls off. Jeff climbs onto the crane to repair the beam, and when the crane swings loose and hits him, Jeff is tossed, unconscious, into the water. Sam dives in to rescue him, thus winning the men's respect. Later, Sam visits Jeff in the hospital, and Jeff interrogates him about his new family, but Sam insists that he is telling the truth. After Sam leaves, Lea confesses to Jeff that she thinks she is still in love with Sam. On his next shift, Sam examines the rig, trying to determine what caused the accident. After Lessing confronts him, Sam tells Miller that he thinks Lessing may one of the saboteurs. That evening, Miller notifies Sam that he has discovered that Royall and Lessing are the Nazi saboteurs and are planning to sabotage the plant the day after the carrier docks. Later, at the plant, Sam finally wins Kelly's respect and Kelly buys toys for the children and offers to chauffeur Sam to work. Miller then tells Sam that Simms is also involved in the plot, and to his horror, Sam realizes that Simms once saw him in a German prison camp in Berlin. After being told that one of his children is sick, Sam hurries home and learns that Miller has phoned Jill with the news that Simms has recognized him. Soon after, Simms appears at the house and, at gunpoint, forces Jill and Sam to accompany him. Sam overpowers Simms, however, and Jill strikes him over the head with a fireplace poker, killing him. Simms's death propels the saboteurs to take immediate action. Convening a meeting that night, Brownell introduces himself as Gestapo agent Colonel Von Browning and announces that he has been appointed their new leader. Brownell proclaims that the group must stay together until daylight, the time of the planned explosion. Desperate, Miller sneaks into a backroom to phone Sam, but is discovered by Brownell and killed. Hearing Sam's voice on the telephone, Brownell leads his men to Sam's house to eliminate him. Mistaking Kelly, who has come to pick up Sam, for their prey, the spies shoot and kill him. Jill cradles Kelly in her arms, and with his dying breath, he identifies Brownell as one of his assailants. Alerted by Miller, Sam has gone to the plant to meet Thane and apprehend the saboteurs before they can act. After Thane arrests Royall and Lessing, Jill runs into the yards and warns Sam that Brownell, who is still free, is their leader. Sam finds Brownell just after he has set the detonator for the explosives. As the minutes tick away, Sam struggles desperately with Brownell. Finally drowning Brownell, Sam disconnects the bomb, thus saving the shipyard. Later, Jeff and Lea are married and Sam explains that he was unable to answer his dying mother's letters because he was a prisoner in a German camp. Upon returning home, Sam sees the children's suitcases packed and asks them to stay with him. Finding Jill in tears, Sam, now assigned to Combat Intelligence, proposes to her and tells her that he wants to adopt the children, and in reply, she kisses him.