Hillbilly private Snuffy Smith competes with Sergeant Gatling at target practice at Army training camp, and loses only by a narrow margin. Snuffy is resentful of Gatling because the sergeant used to be a revenue agent and destroyed Snuffy's stills in the Smoky Mountains. While at camp, Snuffy has been receiving letters and gifts from a pen-pal, who has "adopted" him as her soldier. Although Snuffy cannot read or write, his friend, Corporal Jim Bruce, has been corresponding on his behalf. When his pen pal asks him for a photograph, Snuffy sends her a picture of the sergeant. Shortly after, Gatling is assigned to a detail in the Smoky Mountains, where a new type of radio-controlled rocket will be tested for the military. The colonel insists that Gatling take Snuffy along, so Gatling sends him on a mule-drawn cart. When Snuffy finally arrives at the Saddleback Lodge, he discovers that his cousin, Barney Google, is one of the primary investors in the new rocket. Unknown to Snuffy, his pen pal is really a group of Nazi spies, who, upon receiving his letter stating his new posting, see an opportunity to steal the plans and scuttle the rocket, which is being built by Professor James. To this end, they send Marlene Zara, an attractive blonde, to seduce Snuffy. Based on the photograph, Marlene initially mistakes the sergeant for Snuffy, and when she sees bottle-nosed Snuffy, she decides to seduce the sergeant anyway. When the sergeant bets Barney that his mule, P-40, could outrun Barney's thoroughbred, Sparkplug, Barney uses this as a way of raising funds for an integral part of the rocket he now needs. The spies also see the contest as an easy way to get the rocket, and stake $500 against Barney's half-interest in the rocket. Although they attempt to sabotage the race by drugging Sparkplug, Snuffy distracts P-40 with an attractive female mule, and Sparkplug wins the race. Jim, in the meantime, has fallen in love with Professor James's daughter Julie. When Snuffy gives James some whiskey from his still instead of water, the drunken professor becomes temporarily sidetracked and draws a design for an improved still. The sergeant brings him to his senses and takes the drawings, which Marlene steals in the belief they are the rocket plans. With the rocket test impending, Snuffy and the sergeant are assigned to guard the rocket. Gatling, who is completely unaware of the presence of spies, becomes suspicious when he is besieged by mysterious events, which are actually the spies attempting to knock him out, and stows Snuffy inside the rocket for security. However, the spies launch the rocket with Snuffy inside, and when he finally manages to crawl out of the hatch, he rides through the sky on the rocket as if he were riding a horse. The soldiers and James struggle with the spies, and after Snuffy has been in the air for some time, James finally brings the rocket to the ground, and Snuffy lands safely. Although the launching was unplanned, the military personnel are impressed by the rocket's performance and promise James they will purchase the designs. Julie then captures Marlene, who tries to slip away unnoticed, and the rest of the spies are also caught. Although Snuffy blames Gatling for his unexpected flight, he consents to having his photograph taken seated next to Gatling on the rocket. However, a near-sighted James accidentally pushes the launch lever, and Snuffy and Gatling are sent airborne.