Lord Terence Datchett, Stewart Granger, believes women are vain, trivial and dull. He is irritated when French film star Colette Marly, Edwige Feuillère, arrives in London and takes the table in a restaurant where he wanted to sit. He is scathing of her claims in the newspaper that she is tired of publicity and of men pursuing her, believing it to be an attempt to get more attention. He predicts that if she were really left alone she would throw herself at the first man she met. After being challenged by a man at his club, Datchett decides to invite her to stay at his house, aiming to stage a “scientific experiment” and prove his theory.
Marly is weary of the demands of her publicity-seeking agent and being besieged by autograph hunters and journalists. When she receives Datchett's invitation to stay at his house, she accepts in the hope of some solitude. Arriving at his country house with her maid Clair, Jeanne De Casalis, she is greeted by Datchett's butler Jameson, Ronald Squire, and Lord Datchett who pretends to be Henry Dodds, Datchett's estate manager. The other staff have also been informed to help maintain the deception.
Datchett tries to discover more about Marly, but she is initially unforthcoming. Slowly they become friendly, after they go riding and when Datchett contrives for them to get locked in a cellar for several hours and get drunk on brandy. Clair meanwhile flirts with both Jameson and Patrick, provoking them into jealousy and rivalry. Patrick, W.A. Kelly, the Irish gardener, adds to the tension with his own attentions.
When Reverend Meadows arrives at the house with a christening party for whom Datchett had previously agreed to be a godfather, Marly accidentally discovers Datchett's true identity. Her first angry reaction is to prepare to leave, but then she decides to play along with Datchett's pretense, but intending to teach him a lesson. While out boating on a lake, she pretends to be in distress so that he can rescue her, but when he is knocked unconscious, she becomes the rescuer, but makes him believe he did it.
Datchett's elderly mother Lady Datchett arrives, and he persuades her to join in the ploy and she tries, but without much success. Eventually, Datchett admits he is in love with her and Marly triumphantly reveals that she knows who he really is and what she has done. He goes away, crestfallen, but she then realises she is in love with him.
At the suggestion of Clair, Marly stages a drowning for a second time. Datchett, about to return to London after admitting the experiment was a failure, hears her cries for help and goes to rescue her, but once again, she ends up rescuing him. Safely on the bank, they confess their mutual love and embrace.
