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Midwestern teenager Barbara Ohmstead arrives in Mexico City and is disappointed when Corporal Phil Vaughn, who is flying in from Panama to marry her, fails to appear at the train station.  At the American Consulate, Barbara learns from David Flanner, the overworked vice consul, that Phil's flight has been delayed until tomorrow.

Moments after Barbara leaves David's office to find a hotel, however, Phil shows up, having caught a last-minute military flight.  Phil, who tells David that he fell in love with Barbara when she accidentally dove on top of him in a swimming pool, determines to find and marry her before his two-day pass expires.

Barbara, meanwhile, has no luck securing accommodations and seeks help from David, who is attending his fiancée Rachel Mendoza's harp recital.  Before David can say no, an exhausted Barbara faints from hunger.  Feeling sorry for the teenager, David takes her to a nice restaurant and is seen dancing the jitterbug with her by Rachel's conservative father, Don Gaspar.

Embarrassed, David rushes Barbara to his hotel, but because no rooms are available, she decides to go to Xochimilco, where she and Phil were supposed to spend their wedding night.  Concerned for Barbara's safety, David accompanies her there, and the two enjoy a romantic boat ride together.

When David suddenly remembers he has a dinner date with Rachel, however, he falls into the water and must replace his soaking wet suit with traditional Mexican attire.  David then hides to avoid being spotted by a friend of Rachel's, and Barbara, unaware of the friend, angrily runs off.  David chases after her and is slugged by Phil, who has just arrived and mistakes him for a masher.

After they spend the night with David in his hotel room, the would-be newlyweds rush to the courthouse, but are told they need a health certificate to marry and will have to wait to get one as all of the doctors in Mexico City are attending a conference.  Undaunted, Barbara and Phil go to the conference and try to explain their predicament to a doctor, but as he speaks only Spanish, he has no idea what they are requesting.  When Barbara's subsequent insult is misinterpreted as a compliment, however, the flattered doctor signs their certificate.

Armed with the certificate, Barbara and Phil return to the courthouse, but are informed by an enterprising Mexican that they need four witnesses.  After paying for their witnesses, the couple finds a willing judge, but he tells Barbara, who falsified her age on her tourist card, that she is too young to marry without parental consent.  At their wit's end, the couple goes to Rachel's family estate, where David is entertaining visiting American politicians.  David, whose romance with Rachel is suffering because of Barbara's persistence, promises to help the couple marry outside the country and sends them on their way.

The impetuous Barbara, however, argues with Phil and returns to David's garden party.  Although David tries to convince Barbara to rejoin Phil, who is hiding in a tree, she refuses to listen and goes for a swim in the Mendozas' pool.  When she accidentally dives on top of a passing David, she is momentarily knocked out and awakens madly in love with the diplomat.  Now desperate, David finds Phil and sends him to Barbara's room.  Barbara, however, is still in love with David and breaks her engagement with Phil.  After a heartbroken Phil informs David of his "victory," David threatens to spank Barbara, who screams and brings Phil running.  Phil slugs David and throws Barbara into the pool, then dives on top of her.  Smitten once again, Barbara finally walks down the aisle with Phil, while David and a forgiving Rachel reunite.