Salty Conners and Sparks Johnson, two sailors assigned to a Naval weather observation station on a lonely South Pacific island, have worked out a scheme to earn extra money. They attach notes along with the name and photo of their good-looking superior officer, Lt. Steven F. Smith, to the station's weather balloons, requesting pen pals. Hundreds of women retrieve the notes and respond generously with friendly letters and homemade goodies, which Salty claims as he sorts the stations' mail, while leaving Steve in the dark about his following of well-wishers. Salty and Sparks then sell the homemade treats to the other sailors at exorbitant prices. In spite of the financial success of their scheme, Salty and Sparks have been evading C.P.O. Mike Donovan, who is pressing them to repay a gambling debt. Later, when their unit is ordered stateside, Salty and Sparks pack up the station's materials and hide their savings and the secret pen pal list inside a trunk containing weather reports. When the ship docks in San Francisco, a mob of love-hungry "pen pals" greets the bewildered Steve. Witnessing this reception is Steve's fiance, Betty Lou Prentice, who immediately breaks their engagement. As Betty Lou's father, Senator Prentice, is heading a committee that is threatening to slash Navy appropriations, Steve's commanding officer, Comm. Gerrens, urges Steve to make it up to Betty Lou. To provide an opportunity for reconciliation, Gerrens orders Steve personally to deliver the trunk of reports to Washington, D.C. via the same train on which Prentice and Betty Lou are riding and assigns Mike to assist him. Salty and Sparks give up their week of liberty and also board the train, hoping to retrieve the list and money before their scheme is discovered. However, their efforts to get into the trunk are impeded by their need to avoid Mike, who would confiscate their money, and a bratty child, Bertram, who thinks there is candy in the trunk. Meanwhile, a sultry Russian spy, Sonja DuBois, who believes the trunk contains top secret government information, pursues Steve and has him firmly in a clinch when Betty Lou walks by and sees them. Feeling guilty about their part in Steve and Betty Lou's breakup, Salty and Sparks try to talk to Betty Lou, but cannot get past Prentice, who is suspicious of anyone in a Navy uniform. Needing civilian clothes, they steal another passenger's suitcase, which, to their dismay, turns out to belong to a woman. Undaunted, they change clothes and present themselves to the Prentices as "Mabel" and "Myrtle." After sending Prentice out on the pretext of finding "Myrtle's" purse, Salty and Sparks convince Betty Lou to give Steve another chance. However, their attempt to change back into uniform is hampered when they are barred from the men's dressing room. Meanwhile, the overbearing Mrs. Pursell, owner of the suitcase, has initiated a search for her belongings. When she and the porter find the suitcase under Mike's chair, where Salty and Sparks inadvertently put it, she tries to have Mike arrested. At the next train station, Bertram attempts to abscond with the unattended trunk, but Sonja and her cohorts, Ivan and Peter, seeing the small child dragging the large trunk, steal it and head away. Finally back in uniform, Sparks and Salty spot the spies with the trunk and pursue them. Mike, who has evaded arrest, sees Sparks and Salty leave and reports it to Steve, who then delays his elopement with Betty Lou to instigate a search for them. When the spies realize they are unable to "decode" the military secrets that they believe are in the weather reports, they abduct Steve to interrogate him at their hideout. Watching from the street through a window, Salty and Sparks hear the spies telephone for interrogation "specialists" to deal with Steve, then disguised in clothes they find hanging on a clothesline, try to rescue Steve by posing as the interrogators. They succeed in gaining entrance to the hideout, but are taken captive when the real interrogators arrive. On the street outside the hideout, Betty Lou and Mike, who have been looking for the missing sailors, and Prentice, who has been looking for Betty Lou, see them through the window. Realizing the sailors are in danger, they call for the police, who arrive and round up all the spies. Salty and Sparks tell Prentice that Steve successfully set up a trap for the spy ring, so Steve and Betty Lou's romance, as well as the Navy, is given Prentice's blessing. Later, the Prentices and the sailors continue their train journey to Washington.