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While returning from their three-day leave in New Orleans, Corporal Larry Dolan and his friend, Private Stanley Wenaslawsky, meet fellow soldier Meredith Bixby on the train to Camp Calhoun.  Though he initially seems quite normal, Bixby proves a walking disaster, as he accidentally pulls the train's emergency brake while trying to unscrew a light bulb.  Hoping to make friends with his fellow soldiers, Bixby allows Larry and Stan to oversleep, but the three miss their stop and have to hitchhike ninety miles back to their base from Memphis.

At Camp Calhoun, Major Shelton, a psychologist in the Women's Army Corps, tells her commander, Gen.  Vanderlip, that the well-meaning Bixby has been a constant failure throughout his military career, causing Vanderlip to refer to him as "a sad sack."  Arguing that that the Army is filled with "Bixbys," Shelton receives special permission to study the bumbling private.  Assigned to the same company as Larry and Stan, Bixby makes a poor first impression on Sergeant Elmer Pulley after he accidentally dumps a load of gravel on his new supervisor.  When Shelton asks him to tend to Bixby personally, Larry agrees to take on the assignment, seeing it as a way to avoid any strenuous duty.

On the pretext of studying riflery, Larry and Stan then take Bixby to a roadhouse, and the three end up in a brawl with some local thugs.  Quickly disguising himself as a military policeman, Bixby gets his new friends out of one mess and into another when he mistakenly returns them to a WAC barracks.  Caught by Sergeant Hansen the next morning, the three soldiers are forced to get dressed in front of their female compatriots.  After being returned to Pulley, Larry and Stan are informed by their sergeant that, as punishment, they will not be joining their company on its foreign assignment in Morocco.  Shelton overrules Pulley's decision, however, as she wants Larry shipped out of Calhoun as soon as possible because she is falling in love with the conniving corporal.  In turn, Larry decides to keep his promise to Shelton and gets Bixby qualified on the rifle range, even though the inept private inadvertently shoots a tire out on Vanderlip's passing car.

A proud Bixby then re-enlists and is sent to North Africa with the rest of his squad.  While on leave in Morocco, Bixby, Larry and Stan stumble into a nightclub owned by Ali Mustapha, the ringleader of a gang of thieves who have been stealing U.S. military supplies.  The club's featured performer Zita soon falls for the innocent Bixby, who, in turn, offers to give the singer his life savings so she can go home to Mexico City.  Suspecting the worst of Zita, Larry steps in and lies to Bixby that Zita attempted to romance him as well.  The heartbroken Bixby then goes AWOL and mistakenly wanders into Ali's headquarters on his way to join the French Foreign Legion.

Told that Ali heads a secret force with the Legion, Bixby agrees to help the thieves assemble a stolen R-2 rapid fire cannon at their secret desert hideout.  Two days later, Bixby overhears the thieves plotting his murder, but refuses to leave the camp until he can disarm the R-2, even though Larry, Stan and Zita arrive at the hideout to rescue him.  The three soldiers are soon captured by Ali's men, but a still-free Zita helps them escape the Arabs' dungeon.  The Americans then capture Ali and his men, marching the thieves across the desert and into the custody of the French government.  Bixby, Larry and Stan are awarded the Foreign Legion citation for gallantry, though Bixby disrupts the ceremony when his boot heel melts onto the accelerator of his jeep.  Larry is reunited with Shelton, who informs Bixby that the absent Zita has left Morocco for his hometown of Scranton, New Jersey.

An excited Bixby then decides to show a new private how to hold a rifle correctly and ends up shooting the champagne glasses out of the hands of Vanderlip, a French general and an Arab chieftain.  Realizing only Bixby could have done such a thing, the general assigns the private to the one duty at which he can harm no one but himself: peeling potatoes.