After their initial success with atomic power, scientists in the United States secretly launch the XGMI, the first atomic-powered guided missile. Soon after the launch, three foreign agents, Otto, Stefan and Hogar, seize radio control of the missile, but lose their navigational ability when their equipment malfunctions. After XGMI crashes in the Nevada desert, the spies, hoping to retrieve the rocket for their country, hurry there. Posing as uranium prospectors, they board the rickety “Carolina Cannonball,” a steam-driven streetcar that runs along a spur between a deserted whistle stop at a remote railroad junction and the town of Roaring Gulch. Operating the trolley are Judy Canova and her speed-demon grandfather, Rutherford Canova. When the spies arrive in town, they discover that Judy and her grandpa are the sole remaining inhabitants of the former gold mining boomtown, which is falling apart from neglect. After settling into a dusty room in the town’s dilapidated hotel, the spies tell Judy that they are expecting a shipment containing their prospecting equipment. That evening, as Judy pines for love, Grandpa, heartened by the arrival of prospectors, anticipates the town’s revival and promises that she will soon have a tall, strong and brown-eyed sweetheart. On their next trolley run to the junction, Judy encounters the tall, strong and brown-eyed Don Mack, who claims to be a prospector, but is really an undercover government intelligence agent pursuing the spies. On the ride to Roaring Gulch, Don’s baggage containing his walkie-talkie is crushed, severing his contact with headquarters. In town, Don tries to warn Grandpa about the spies, but the hard-of-hearing older man misunderstands him and thinks he is expressing his admiration for Judy. Don then tries to talk to Judy, but the spies show up and, to avoid being seen by them, he leaves abruptly without getting his message across. After unpacking their equipment, the spies steal the “Carolina” to search the desert for XGMI, but when the trolley’s boiler explodes, they must walk back to town. Upon discovering that their room has been searched, they realize Don is a government agent and capture him with Judy’s misguided help. Believing that Don is a thief and that the men just want to help him reform, Judy leaves Don alone with the enemy agents. After she departs, they begin to interrogate Don, but Otto’s fumbling attempts backfire and the spy confesses more than his prisoner. Outside, Judy and Grandpa discover that the “Carolina” is missing and follow the track to where it sits, broken and abandoned. While Grandpa begins repairs, Judy searches the nearby desert for the trolley’s pieces and finds the XGMI, which she mistakes for a wing tank of an airplane. The resourceful Canovas then use the XGMI to replace the boiler and Judy fashions discarded pieces of the rocket into a bracelet. With more vigor than before, the “Carolina” takes them back to town, where the spies’ equipment, a radiation detector, registers the XGMI’s proximity. However, readings from the equipment become garbled in the presence of Judy’s bracelet, which causes lights, pinball machines and the player piano to act bizarrely. When the spies eventually learn from the gullible Judy that the XGMI now serves as the trolley’s boiler, they tie her up next to Don and hijack the vehicle with Grandpa still on board. Motivated by the threat to national security, Don and Judy quickly untie themselves and jump on the “Carolina” before it leaves town. As the atomic-powered “Carolina” speeds along the track at a frightening rate, a fight ensues, but Don and Judy eventually overcome the spies and tie them up. Unable to stop the rocket-fueled trolley, Don fears that it will increase speed until it blows up and kills innocent bystanders. After the “Carolina” leaves the spur and races toward Las Vegas on a main railroad line, Don writes a note of warning and throws it to the bewildered stationmaster at a small whistle stop along the route. The stationmaster relays the message to the proper authorities and soon Air Force planes fly above the “Carolina,” dropping bombs to blow it off the track before it reaches the populated area. Unsupervised by their captors, the spies free themselves and force Don and the Canovas to jump off the racing trolley. Afterward, the bombers hit their target and the “Carolina” and its occupants are blown to bits. Later, in Don and Judy’s presence, Grandpa demonstrates to the Atomic Power Commission a working model of the “Carolina,” which has been declared the first atomic-powered streetcar and proof that the country has entered the atomic age.