A jealous husband tests his opera-singer wife's fidelity by pretending to be another man.

Viennese musical comedy stars Karl Lang and Maria Lanyi are successful in their careers, but their six-month-old marriage is not doing well. When Maria expresses an interest in pursuing a career in Grand Opera, Karl thinks that it is just an excuse to leave him, and confides in their good friend, Bernard Fischer, that he thinks Maria is simply in love with romance and will soon be seeking other men. Bernard's suggestion that Karl act like "Tannhäuser" and be romantic toward Maria backfires when Maria becomes annoyed at Karl's late-night singing. After their performance the next night, Karl and Maria go to a nightclub, and Karl receives a telegram. Although Karl had sent the telegram to himself, he claims that it is from a friend who needs to be bailed out of jail. After he leaves, while Maria is confiding in Bernard that she, too, is jealous, Karl dons the disguise of a bearded Russian guardsman named Vassily Vassilievitch, then performs for the audience. Maria is intrigued by the singer, and when he comes to her table, they begin a flirtation. Shortly after "Vassily" leaves, Karl changes clothes again and returns as himself. The next day, when flowers arrive for Maria, she and Karl argue because she denies that the gift came with a card, and merely says that the man who sent them loves her. Later, she confides in Bernard that she thinks it was Karl posing as Vassily and was overjoyed to read his romantic note suggesting that she stand by her window at five that afternoon. Because it is Sunday and they have no performance, Karl must make an excuse to leave and says that he has a charity function to attend that will require him to be away until late the next day. At 5:00 o'clock, Maria goes to the window, as the note suggested, and Karl, who is standing below, as Vassily, is upset to think that she is contemplating being unfaithful. Unknown to him, though, Maria has drawn whiskers on one of his photos and confirmed her suspicions that the bearded Vassily is really Karl. When "Vassily" accepts her invitation to come up to the apartment, Karl arrives in his costume, amusing Maria, who notices that his disguise does not even fool their dog, whom Karl cannot discourage from licking him. While Maria and Karl engage in some verbal love-making, she tells him that she likes strong men, and he pounces on her. At first she angrily pushes him away and tells him to leave, much to Karl's delight, but then flirtatiously tells him that her husband will be out that evening. Because Karl cannot take any more, "Vassily" leaves, then Karl quickly returns, explaining that he had missed his train. Before he leaves again, they argue and he accuses her of seeing another man. Later that night, "Vassily" makes another appearance and serenades her, after which he and Maria go to a gypsy café. "Vassily" tells Maria that he feels sorry for her husband, then she passionately kisses him and runs off. Unknown to Karl, Bernard has been hiding in the bushes observing their conversation and tells Karl to follow Maria, otherwise he will never know whether Maria is faithful. Karl does follow Maria, who tells him they must say goodbye. Later, though, she throws her key off the balcony. The next day, Maria is very happy and smug, worrying Karl, who decides to appear during their performance of The Chocolate Soldier dressed as Vassily. Although the audience does not understand why he is dressed as a Russian guardsman, Maria is happy that he is finally revealing himself and offstage tells him that his kisses gave him away because no man on earth kisses like him. Karl is now happy and secure in Maria's devotion, but becomes a bit worried when they return to the stage and she winks at someone in the audience.