Based on a play by Melchior Lengyel, the wife of a British diplomat meets a man in Paris who is entranced by her, and he knows her husband.
In a Paris hotel Maria Barker (Marlene Dietrich) registers as Mrs. Brown. She visits an old friend, Grand Duchess Anna Dmitrievna (Laura Hope Crews), and meets Anthony Halton (Melvyn Douglas), who mistakes her for the Duchess. They dine together privately, but she refuses to give her name. Anthony calls her Angel and says he loves her. They kiss. When he buys violets for her, she disappears. Diplomat Frederick Barker (Herbert Marshall) returns from Geneva and finds Maria in his bed; so he uses hers. Graham (Edward Everett Horton) gives her a telegram, and she wakes Frederick, saying she had a dream of him. Frederick is upset by the telegram.
Their butler Walton (Ernest Cossart) is getting married. Frederick and Maria try to quarrel but disbelieve each other's affairs. At the races Maria gets upset and leaves. At a luncheon Anthony meets Frederick. Anthony says in the war they both loved the same woman in Paris. Frederick tells Maria that Anthony lost his head over a woman at the Duchess' and is looking for her. At the opera Frederick tells Maria that Anthony will lunch with them. Anthony tells Frederick he admires him, and they discuss Anthony's love. Maria pretends not to know Anthony. The servants notice that Maria and Anthony did not eat. Anthony plays piano. Frederick steps out, and Anthony calls her Angel; but she says he is confused. She asks him not to come back there anymore and to forget it. He asks why. Frederick comes back, and Anthony stays for a drink. Maria asks Anthony to describe the lady; he says she has brown eyes and brown hair. Alone, Anthony tells Maria he will wait for her in Paris. Maria wants to go to Vienna with Frederick, but he says he has to go to Geneva. Frederick learns he can charter a plane to Paris like the one Maria took. Maria asks Frederick to let her go to Paris. Frederick calls Anthony and hears him playing the song on piano.
In Paris Frederick calls on the Duchess, who tells Anthony to wait longer. Frederick asks the Duchess to describe the lady Angel and offers a donation. The duchess goes out, and Maria comes in. Frederick wants to know if the lady is blonde. Maria says she looks like her. Frederick says she is Angel, but she denies it. Frederick checks as Maria tells Anthony it is over with Frederick, who comes back and says he met Angel. Frederick asks Maria to go to Vienna and walks out, accompanied by her.
The Lubitsch touch gives this drama subtlety, but it lacks dramatic power and humor. Ironically a celebrated diplomat is almost fooled by his wife, reflecting the dangerous pre-war atmosphere in Europe.