On a plane to Mexico City, American nightclub entertainer Angel O'Reilly slips a stolen diamond into the pocket of her airsick seat mate, banker Tom Grant.  Tom is detained in Mexico City when customs officials find the famous diamond in his pocket, and Angel breaks off her engagement to Boris Cassall, who coerced her into transporting the diamond across the border.  Angel slips away from Boris, who has taken her wallet to keep her from escaping, and is driven into the city by kindly taxi driver Pablo, who dreams of being an opera singer.

After Angel fails to get work on her own, Pablo tells the owner of a posh nightclub that she is the Condessa of Costa Mora from Spain, and she is hired to perform.  Angel is horrified when she sees Tom at the nightclub the first time she sings, but Tom insists that she join him, as well as his estranged wife Helen, and Helen's suitor, Mexican bullfighter Manolo Segovia, for drinks.  Pleased that Manolo has become smitten by Angel, Tom reveals that he is aware that she framed him, and insists that she maintain her pretense as a countess and lure Manolo away from Helen, who has left Tom for the bullfighter.  Tom underwrites Angel's luxurious hotel room and expensive clothes, and Angel succeeds in making Helen extremely jealous.

One afternoon, Boris sees them at the bullfight, and learns that Angel is pretending to be a countess.  Later, Tom invites Angel to his country home, where Helen is planning to premiere a ballet she has created.  Helen discovers that Angel is an impostor after her houseguest, Irene Denny, claims to know the condessa's best friend.  Just as Helen is about to announce publicly that Angel is a fraud, she is interrupted by the arrival of Boris, who announces himself as the Condé of Costa Mora.  As this seems to confirm that Angel is the real thing, Helen drops her accusation.  Manolo then intensifies his romantic pursuit of Angel, but Angel falls in love with Tom because he has been honest and supportive.  Angel has no intention of marrying Manolo or Boris, and reassures Tom that she was not a willing participant in the jewel theft.

On the eve of the ballet's premiere, Tom pays Boris to leave for good.  Before going, however, Boris steals Helen's diamond necklace while proclaiming his love for her.  Angel finds them together and, disgusted by Helen's loose morals, decides to leave with Boris.  After Helen discovers that her necklace has been stolen, Manolo follows Boris and Angel, chasing their car off the road and knocking out Boris.  Manolo then retrieves the necklace and Angel tells him her true identity on the way back to the Grant mansion.  Although Manolo still wants to marry her, Angel does not love him.  Helen, meanwhile, tells Tom she is definitely suing for divorce despite his efforts to revive their relationship.

After Manolo returns Helen's diamonds, Tom has renewed hope for the future, and goes to see Angel while Manolo flirts with a pretty dancer.