On a Sunday afternoon in London in June of 1938, Mr. Hilary Ames frets because he has been unable to get a plumber to fix a stopped-up sink, and it is now one-half hour before his planned cocktail party. When a stranger, Adam Belinski, arrives, Ames mistakes him for a plumber, but Adam quickly informs him that he came to borrow money from an absent friend from whom Ames is subletting. The attractive Cluny Brown then arrives, announcing that she is the niece of a plumber Ames had called, who is unavailable at the moment, and that she would like to have a try at fixing the sink. As she works, she complains to Adam that her uncle Arn always rebukes her for not knowing her place. Adam replies that one's place is wherever one is happy, and that happiness is a personal adjustment to environment, even if it is as radical as "feeding squirrels to the nuts." After the drain is fixed, Cluny celebrates by accepting Adam's offer of cocktails. When Uncle Arn then arrives and finds Cluny drunk, he indignantly informs her that she is going into service as a domestic. After they leave, Adam pockets the payment that Arn refused. Later, during the party, Adam passes out drunk, and awakens to discover Andrew Carmel, the naïve scion of a wealthy family, looking at him in awe. Andrew explains to Betty Cream, a blasé socialite with whom he is in love, that Adam is a great Czech professor and writer, and one of Hitler's worst enemies. When Adam, who is facing eviction for non-payment of rent, mentions that he is a man without a home, Andrew, thinking that Adam means that he is a political refugee, takes pity, gives him some money and invites him for a drink. Soon after, Cluny goes to work as a parlor-maid at the Carmel's country manor and is invited for tea by the Carmels, Andrew's parents, Henry and Alice, when she is mistaken for a guest of their neighbor. After they learn that she is their new maid, they politely but abruptly leave her to the butler, Mr. Syrett, and the housekeeper, Mrs. Maile, who instruct her as to the proper etiquette of a servant. Meanwhile, Andrew has arranged for Adam to be a house guest at the manor, as he fears Adam is in danger from the Nazis in London, while he, Andrew, returns to London to pursue the flighty Betty. That night, when Cluny serves dinner, she drops the tray upon seeing Adam and exclaims "squirrels to the nuts!" Although the Carmels want to dismiss Cluny for her unseemly behavior, Adam talks them out of it. Afterward, a tearful Cluny is comforted by Adam's declaration that he, too, feels out of place there, and Cluny then gratefully embraces him and vows undying friendship. Cluny soon becomes enthralled with the prospect of becoming the wife of the town's pompous chemist, Jonathan W. Wilson, who lives with his disagreeable mother. Cluny confides to the crestfallen Adam, who harbors romantic feelings for her, that as an orphan, she hopes to find her place in life as Wilson's wife. Soon after, Andrew arrives at the manor after a row with Betty only to find that she has been invited for the weekend by his mother. During Mrs. Wilson's birthday celebration that evening, Wilson is about to announce his engagement to Cluny, when awful sounds emerge from the bathroom, and Cluny impulsively offers to fix the pipes. Mrs. Wilson and her guests are aghast at Cluny's behavior, and once they are alone, Wilson rebukes her. Later that night, Adam enters Betty's bedroom in his dressing gown, ostensibly to plead Andrew's case. Betty, sensing Adam's desire, orders him to leave, and when he refuses, she screams nonchalantly and continues to read her book. Andrew is incensed to see Adam leave Betty's room, but when Adam says he mistook her door for the bathroom, Betty corroborates his story and claims that she thought he was a burglar. Lady Carmel then has a talk with Betty, who admits that she plans to marry Andrew and agrees to finally tell him so. The next day, Henry tells Adam that Andrew has grown into a man overnight, and that he plans to join the R.A.F. because of some man named Hitler. After Adam explains who Hitler is, Henry is filled with fatherly pride. When Betty announces that she and Andrew are engaged, Adam, who now plans to leave, offers congratulations, but Andrew, not believing his story of the previous evening, challenges him to a fight. Betty forestalls the brawl, however, by informing Andrew that they owe their engagement to Adam's impetuous behavior. Before leaving, Adam learns that Cluny is not feeling well and so asks Mrs. Maile to deliver his farewell gift to her. Cluny pursues Adam to the train to thank him, and when she relates her disgrace at Mrs. Wilson's birthday party and renounces her foolish behavior, Adam orders her onto the train, and once inside, he throws her servant's cap and apron out the window. He then proposes, and to support his new wife, Adam revises his plans to write a book on morality and expediency, but instead moves to New York and authors a series of murder mysteries which become best sellers.
