In Heidelberg, French army captain Henri Rochard seeks out WAC lieutenant Catherine Gates and, when he finds her, returns a collection of underwear. Deeply embarrassed and angered, Cathy follows him and demands that he come back to her office and explain to her fellow WACs that their laundry was accidentally switched. Henri refuses and the two continue the quarreling that has characterized their relationship since their first mission together. Henri informs Cathy that he is in Heidelberg to meet the WAC who will accompany him on his next mission, but soon learns that his prospective partner has been reassigned, and Cathy will again be working with him. Next they discover that the only available transport is a motorcycle with a side car, and because only Cathy has been cleared to operate the machine, Henri will have to ride in the sidecar. After several mishaps, including a road block, a near plunge over a waterfall, and losing their way, Cathy and Henri arrive at their destination, Bad Nauheim. At the hotel, Cathy complains of back pain, and Henri offers to give her a back rub. After she falls asleep, Henri tries to leave her room, but discovers that the door handle has fallen off, trapping him inside. Henri spends an uncomfortable night in a chair, and in the morning, Cathy refuses to believe his story. Unknown to Henri, the innkeeper's wife has replaced the knob, and so, when he again tries the door, it opens easily. Eventually, the innkeeper's wife explains everything to Cathy, but not before Henri falls off the roof while trying to keep his presence in Cathy's room a secret. Later, Henri disguises himself to search for Schindler, a black market lens grinder. He refuses to let Cathy help him, so she has breakfast with a fellow officer. From him, she learns that the black market is about to be raided, and when Henri asks her to vouch for his identity, she follows his earlier orders not to reveal that she knows him, and allows the police to arrest him. While Henri is in jail, Cathy finds Schindler, who happily agrees to leave Germany and ply his trade in France. Later, she apologizes to a furious Henri, and by the time they return to Heidelberg, they have fallen in love. After a great deal of red tape and interference by well-meaning friends, Cathy and Henri are married in three different ceremonies. Before they can consummate the marriage, however, Cathy is ordered back to the United States. They subsequently learn that the only way Henri can get a visa to emigrate with her is under the War Bride Act, as a spouse of a member of the expeditionary forces. After many misunderstandings, Henri is granted permission to sail for America with Cathy, but before they leave, circumstances and Army regulations conspire to keep them from spending the night together. Finally, in order to get past unbelieving Navy officers, Henri must dress as a woman. The deception works, but once underway, Henri's disguise is penetrated, and he is arrested. Cathy manages to straighten out the situation, and although he is free to leave the room where he has been imprisoned, Henri invites Cathy in. After he locks the door on the inside and throws away the key, Henri and Cathy finally have their wedding night.