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Determined to win her socialite aunt Carrie Valentin's latest "treasure hunt" challenge, young Monica Strong and her married boyfriend, Harry Rawlinson, set out to find and bring back a truly faithful husband and a woman over twenty who has never had a love affair.  While Harry returns to the party with Lady Cynthia Darrington, an aviatrix with whom he had collided on a country highway, Monica fetches her father, Sir Christopher Strong, a respected London politician and model husband.  After the party, Monica and Harry become good friends with Cynthia, who also grows close to Christopher.

In spite of his love for his wife Elaine, Christopher is drawn romantically toward the single-minded Cynthia, and she, to him.  Although she senses her husband's growing passion, Elaine, who has forbidden Monica from seeing Harry, accepts Cynthia's presence at their summer villa in Cannes.  During her visit, Cynthia and Christopher take a midnight boat ride and confess their love for each other.  At the end of the ride, the couple, whom Elaine sees kissing through her bedroom window, vows to end their relationship and separates.

Soon after, however, Monica shows up at Cynthia's apartment, determined to kill herself because Harry, who has divorced his wife, refuses to marry her because of a one-night affair she had in Cannes.  After preventing Monica's suicide, Cynthia leaves England for New York to participate in a dangerous around-the-world flying competition.  Although she wins the grueling contest, Cynthia longs for Christopher and is ecstatic when he shows at her New York hotel.  There they consummate their love, and Cynthia agrees to give up flying and devote herself to Christopher.

Eventually, however, Cynthia's affair is discovered and denounced by Monica, who is now married to Harry and is pregnant, grows restless and is about to accept a high-attitude flying challenge when she learns that she, too, is pregnant.  Faced with destroying Christopher's marriage and career, Cynthia chooses to remain silent about her pregnancy and, while breaking the altitude record, throws off her oxygen mask and crashes to her death.