THE MAJOR AND THE MINOR (1942). Billy Wilder, already an established screenwriter, decided to direct his first movie in order to have full control of the script that he wrote for it with his collaborator Charles Brackett.
By May 1941, Susan Applegate (Ginger Rogers) from Stevenson, Iowa, has tried everything to survive in New York City . When, in a last-ditch effort, her first new assignment to massage the scalp of lecherous Robert Benchley ("I'd like to slip out of this wet coat and into a dry Martini") means fending his advances, she quits in disgust to return home.
Train fares have risen and she impersonates a 12-year old for a half-price ticket. On the train she arouses the conductors' suspicions, flees to the compartment of Major Phillip Kirby (Ray Milland), a Military Academy instructor, spends the night in his roomette.
Rita Johnson, the Major's fiancee and daughter of the Commandant, coming to the stalled train, has a fit when she finds another woman in the compartment. For an alibi, Milland gets Rogers, called Suzu, (he still thinks she is a child) to spend three days at the Academy, where the cadets act like juvenile wolves.
Rogers shares the room of Johnson's young sister Diana Lynn who is immediately wise to her (she's also opened her suitcase) but befriends her. Rogers has fallen for Milland and he ("Uncle Phillip") is most attracted to the presumed child.
Milland is anxious to get posted to active duty, but underhanded Johnson tries to scuttle this with a letter to Washington. Lynn steams it open and conspires with Rogers to get to the switchboard, impersonate Johnson and ask for Milland's wished-for assignment.
At a formal dance, Benchley turns up as the father of a cadet, slowly remembers Rogers, tells Johnson, who unmasks her, in private. Rogers leaves for Iowa. Soon after, Milland, posted to the West Coast, stops by, insists on visiting Mrs. Applegate (played by Lela Rogers, Ginger's own mother) and Ginger impersonates her. When Milland leaves, she is there, at the the train station, as herself. The (happy) end.