After Richard "Dick" Lennon, a middle-aged rubber magnate and widower, marries young, silly Dolly Jordan, he is criticized by his butler, Murray, and his alcoholic brother Joe. Although Dick maintains that Dolly truly loves him and wants to bear his heir, Joe, an unproduced playwright, is skeptical and uses his drunken, acerbic wit to question Dolly's motives. One evening while visiting Dick and Dolly, Joe meets Louis DeSalta, a South American businessman with ties to the Lennon rubber company. After Joe makes casual conversation about South America with DeSalta, he finds a slip of paper in DeSalta's coat with the name "Louis Winkler" written on it. At the same time, he sees in a hallway mirror a reflection of Dolly and Louis, supposed strangers, embracing out of sight of Dick. The next day, Joe confesses to Anne Cornwall, Dick's devoted secretary with whom he has enjoyed a long platonic love affair, that he wants to marry her. Although Anne admits that she, too, is in love, she refuses Joe's proposal out of respect for his independence. Later, an inebriated Joe attends a dinner at his brother's home and, as Dolly, DeSalta and the other guests listen, relates the plot of his new play. In the plot of Joe's play, a rich widower marries a young gold digger who conspires with her lover to trick the widower into believing that she is pregnant with his child. Before the baby's due date, the gold digger divorces the widower and sues for child support. Stunned that Joe has unmasked her scheme, Dolly screams and faints at the dinner table, but then convinces a still trusting Dick to take her to Paris. At the same time, Joe reveals to Anne that, after Dick married Dolly, he stopped drinking in order to investigate Dolly and save his brother from ruin. Apprised of Dolly and DeSalta's subterfuge, Anne agrees to help Joe trap the couple and calls Dick away from his home on a phony business emergency. Once alone with Dolly, Joe confronts her with her dubious past, which includes two previous rich husbands, and DeSalta's phony identity. Although Dolly denies Joe's charges, DeSalta, unaware of Joe's presence in the room, marches in and unwittingly exposes their plot to trick Dick into divorcing Dolly. After Joe forces Dolly to sign a confession and leave Dick without collecting a penny, Joe breaks the news to Dick. Joe finally convinces his heartbroken but wiser brother to join Anne and him on their honeymoon cruise.