In Berne, Switzerland, when playboy Paul Wagner sneaks into Rosalind Brown's box at the opera, her father lets him know once again that he is not a proper suitor.  Later, while Rosalind cajoles Brown into giving Paul another chance, Paul gets so drunk that he is thrown out of a nightclub.  Despite a hangover, Paul arrives at the Brown house the next day, but when Brown's secretary, Adolf Pumpfel, turns out to be a man Paul socked the night before, Paul is banished from the house.  All of the servants, except scullery maid Katerina Linz, are shown Paul and ordered not to allow him into house, so Paul arranges to meet Katerina.

On Sunday, disguised as a chauffeur, he takes her to a fair, and later sneaks upstairs to see Rosalind.  The next day, when Rosalind sends a note to Paul, it must be delivered by Katerina, because the other servants are occupied.  When Katerina arrives at Paul's, he pretends that "the master" is out, then, when his friend Charlie arrives, Charlie pretends to be Paul's employer.  After Charlie brusquely kisses Katerina, she slaps him, and Charlie "fires" Paul for laughing.

Because Katerina is in love with Paul and feels responsible for his predicament, she uses her savings to buy a taxi for him, then asks him to meet her at the fair on Sunday for a surprise.  Feeling remorseful for his deception, Paul sends her an apologetic goodbye note, but, as she cannot read, she thinks it is a love letter and waits for him.  When he doesn't show up, she goes to his house, and is greeted by Karl, Paul's servant, who tells Paul what has happened.  When Paul finds Katerina, he is touched by her gesture and, when he realizes that she couldn't read the letter, he tells her that it said that he loved her.  That night, Brown goes to Paul and tells him he has changed his mind and will now accept him as his son-in-law and will announce the engagement at a party on Friday.

The next day, Pumpfel shows Brown a picture of Katerina and Paul that he had secretly taken, and Brown conceives of a plan to promote Katerina to parlormaid in time for the party, hoping to break up Rosalind's romance.  Paul shows up late, then, just as he tries to tell Rosalind what has happened, Katerina appears.  Heartbroken, she tells Brown she doesn't know Mr.  Wagner, only his chauffeur, and refuses to listen to Paul.  After apologizing to Rosalind, Paul tries to find Katerina, who has left for the train station, so he tells a policeman that she has taken a brooch from him.

After her arrest, Paul tells the police that she is his fiancée, and, though she says that she couldn't marry a gentleman, especially one who lies, he tells her that he has always told her the truth, even when he didn't know it.