Having gone to Newfoundland to investigate his missing past, Captain America has discovered a secret lab where ex-Nazi spy Lyle Dekker displays a twelve-foot-tall replica of Cap. Dekker reminds him of the first time they met, many years ago during the war….
Steve Rogers and Bucky Barnes were given a furlough in California by the top brass so they could carry out a mission as Cap and Bucky. The task: investigating Axis sabotage on the set of a movie serial, "The Adventures of Captain America." They parachute onto the lot of Democracy Pictures and right into the path of a western chase scene. The rehearsal is halted and Bucky meets his movie hero Sundown Dawson who is equally impressed at meeting him and Cap. Dawson escorts them to the studio where the Cap serial is being filmed; there they meet special effects ace Lyle Dekker, director Whit Spencer, and the star Glenn Reeper. Cap is a bit confused about the changes in his costume, the addition of a gun, and the lack of Bucky in the production. The production has been plagued by mysterious "accidents" and today is no exception. While the visiting heroes look on, an automatic machine gun turns out to contain real bullets. Glenn Reeper is wounded by the suddenly all-too-real gunfire and Bucky pulls him to safety while Cap destroys the gun. As Glenn is rushed to the hospital he asks Cap to take over as the star of the picture. Production continues and the identity of the new lead actor is kept secret from the public, while the saboteur keeps a low profile. Eventually the villain, Lyle Dekker, is contacted by his superior, the Red Skull, and ordered to end the production. Dekker substitutes a real death ray for the prop used in the movie. The next day during filing, the heroine is put in real danger, Cap uses his shield to protect her from the beam while Bucky disconnected the fiendish device. Dekker takes Bucky hostage and drives off in a truck. Cap pursues on a motorcycle accompanied by Sundown Dawson. Cap leaps aboard the running board of the truck and struggles with Dekker and the vehicle runs off the road and into the ocean. Cap dives in and rescues the unconscious Bucky and Dawson lassoes them and pulls them to shore. Dekker is presumed to have drowned…
…until now when Cap finds himself a prisoner of the ex-Nazi who has Cap electrocuted and ready to be put to use in bringing the Ameridroid to life….