Sam Gallagher (Pat O'Brien), a former foreign correspondent and now a United States Government agent, gets a job through his brother Jeff (Chester Morris), whom he has not seen in seven years, in the Seaboard Shipyards as a "pileback" in order to track down a gang of Nazi spies who are plotting to sabotage the shipyards. Jill McCann (Carole Landis),an FBI agent, poses as Sam's wife, and two children, 6-year-old Paul (Richard Lyon), and 4-year-old-Joan (Carol Nugent),complete his "cover family." THe set-up looks fishy to Jeff, and he imparts his suspicions to Lea Damoran (Ruth Warrick, the girl both brothers had courted in the old days. Meanwhile, Sam works hard and makes friends with most of the men in his crew. He gets a line on the saboteurs and one by one, their identities are revealed to him. The Nazis, led by Brownell (Tom Tully)who, in reality, is Colonel Von Braun of the German Gestapo, plan to blow up the yard while an aircraft carrier is docked there.
Just before dying from wounds received in a skirmish with Indians Capt. Forsythe orders his cavalry troop's doctor, Capt. Robert MacClaw, to take command. His men don't like it and think that Sgt. Elliott should have been put in temporary command until they reach the Fort. MacClaw admits he knows little of battle tactics but takes charge only with the promise that he will do the best he can. If anything, the men are embarrassed at having such an inexperienced man leading them and MacClaw agrees not to let on that he's a doctor.