This sprawling, eventful play begins with the death of a certain Mr. Whitefield in early twentieth-century England. We never meet or see Whitefield himself at all—instead, the play's events track the consequences of his death. The first of these consequences comes from a dispute over the guardianship of Whitefield's daughter, Ann. Ann is a clever, relentless, and wildly manipulative young woman, and her father's will stipulates that she be left in the care of not one but two men. One of these men is Roebuck Ramsden, a cautious, respectable family friend who has long acted as a kind of grandfather figure for Ann. The other man, Jack Tanner, is a socialist firebrand and the author of a controversial guide to revolutionary politics. Ann is closer to Tanner's age, and the two are old friends who have had a falling-out. However, when they are left alone, the two flirt wildly. This is particularly shocking because Tanner's close friend Octavius is in love with Ann, and expects to marry her. Meanwhile, Tanner and Ramsden can't stand each other—Tanner thinks Ramsden is a hypocrite, and Ramsden thinks Tanner is obnoxious. Ann, instead of choosing one of these men to be her guardian, insists that they work together to care for her, as her father wished. Ann's mother Mrs. Whitefield is present as well, but she is a somewhat weak-willed person who tends to cave to her daughter's intense behaviors.
To this volatile mix, another character is added: Octavius's sister Violet. While Octavius is a romantic aspiring poet, we soon learn that his sister is a blunt, practical young woman. She arrives at Ramsden's house, where the others are gathered, with the news that she is pregnant. In these conservative times, the news is considered shocking, especially since the others believe that she's pregnant out of wedlock. However, Violet reports, she's actually secretly married. She refuses to tell the others who her husband is, and she refuses to leave the country in order to hide her pregnancy. The others are left in the dark, but the audience soon learns that Violet is married to a wealthy American acquaintance named Hector Malone. Both Malone and Violet disappear for much of the second half of the play, returning at the end.
After this initial period of explosive drama, Tanner leaves England with his chauffeur, Henry Straker. Among other things, Tanner, who believes that romance and women will drain his energy and independence, wants to escape Ann's romantic pursuits. In fact, Straker himself, who seems to view the dramas of these wealthy employers with distant skepticism, firmly believes that Ann is after Tanner rather than Octavius and has been doing his best to convince Tanner of as much. Ann herself spins an elaborate series of lies in an attempt to get Tanner to invite her on his trip, but ultimately doesn't succeed, and the two are separated while Tanner goes to Spain.
In Spain, Tanner and Straker are abducted by a somewhat ridiculous crew of would-be revolutionaries led by a charismatic man named Mendoza. Tanner quickly befriends his kidnappers, although, devoted as he is to progressive ideals, Tanner sees that Mendoza has no real interest in socialist politics. While spending the night in Mendoza's camp, Tanner has a vivid dream in which the mundane dramas of life in modern Britain time-travel and become part of a Don Juan narrative. In this particular scene, Tanner himself becomes Don Juan, and finds himself in hell. There he speaks with Ann, who has become an old woman, Ana—the Dona Ana de Ulloa of the original Don Juan story. Since she lives in a more modern and secular era, Ann is a strong-willed woman who goes after men based on her own desires. In the dream, though, she becomes a devout Catholic and something of a rule-follower, and is horrified to find herself in hell. Ramsden, meanwhile, is represented by a statue, who for the most part professes the same stale and judgmental worldviews as the original Ramsden. Finally, Mendoza appears in the form of the devil himself. The devil is nonthreatening but somewhat shallow and uninspiring, and wants to keep Tanner/Don Juan in hell, a comfortable but boring place devoid of beauty. Don Juan, though, wants to see heaven, and departs. Tanner/Don Juan wishes to become "superman," the intellectually and artistically ideal human, and can only do so by leaving behind comfort and pursuing a more challenging and difficult path. The Devil and the Statue both find this goal ridiculous: they do not subscribe to the Nietzschean idea of the constantly-improving man or the essential life-force that allows man to continually improve. When Tanner wakes, the others have come to rescue him: Ann, Ramsden, Octavius, Violet, and Malone. Ann is particularly eager to talk to him and help him escape his captors. However, Tanner makes sure that Mendoza and his men aren't arrested or punished, identifying them as friends and escorts.
In the final act, the characters have retreated to a Spanish villa. At this point, Malone's father—an Irish-American businessman also named Hector Malone—arrives in England. He has found out about Hector Jr.'s romance with Violet because he intercepted a letter between the two. Malone tries to convince Violet that she should not marry his son, because he plans on marrying him off to a high-born English heiress. Many of his motives come from a desire for revenge, since the Irish potato famine devastated his family and country during his childhood. Violet remains calm and cool throughout the exchange, and manages to convince Malone that she's personally worthy of marrying his son, even if he still would prefer an aristocrat. Their conversation is cut off by the arrival of the younger Hector Malone himself. He announces that he is in fact already married to Violet, finally solving that particular mystery for the other characters—Ann, Octavius, Tanner, and Ramsden are in fact looking on. And, Malone Jr. says, he has no need of his father's approval or money. Instead, he plans on working to support himself. The idealists Octavius and Tanner even offer Hector some money while he tries to become financially independent. Hector refuses, basking in his independence, but Violet has less faith in her husband. After the others have departed for various other errands, the older Mr. Malone gives Violet a check, knowing that his son will have trouble making money. In fact, Malone notes with satisfaction, Violet will be a better wife for his son than any aristocrat's daughter.
This leaves the central love triangle unsolved, however, close to the end of the play. Octavius confesses his love to Ann, but she turns him down, saying that her mother wants her to marry Tanner, and that her father instructed her to do so in his will. Octavius, still convinced that Ann loves him, believes that this is an example of her self-sacrificing nature. However, as she departs, Mrs. Whitefield arrives and finds a crying Octavius. She tells Octavius he's been duped, since she never told Ann to marry Tanner. However, she tells Octavius, that might be for the best, since Ann will crush his delicate spirit. Tanner, in the meantime, claims that he has no desire or plan to marry Ann, or to marry at all. He finds Ann to be a bully, and bluntly says so. Octavius is bewildered, meanwhile, asking Ann whether she'd marry a man who doesn't desire her at all, but she explains that, from her point of view, it's a better match. Jack, she says, doesn't have any unrealistic expectations for her to live up to. Octavius, meanwhile, is such a romantic that he'd be happier heartbroken and single than faced with the day-to-day realities of marriage.
Tanner begins to fret that social pressure will force him to agree to marry Ann against his principles. It seems, though, that he also desperately wants to marry Ann. When he objects to her pursuit, she uses his own philosophy to convince him, telling him that the life-force has driven them together. According to Tanner's own theory, the life-force causes women to pursue men for their own needs, and she tells him he's no exception. As they talk, Tanner confesses his love for Ann, and holds her so tightly that she faints. The other characters rush back to center stage to help revive her. When she comes to, she and Tanner announce their plans to marry—although Tanner insists that he is not happy about the marriage, and will keep the ceremony as simple as possible. Ann evidently finds this announcement charming, and Tanner's words are met with lighthearted celebration and laughter as the play concludes.
