The affluent and respectable Roebuck Ramsden, an aging civil reformer, sits in his study. Near him stand stone busts of John Bright and Herbert Spencer, among other luxurious décor that reflects Ramsden’s cultural clout and sociopolitical views. A maid interrupts Ramsden’s contemplations to announce the arrival of Octavius Robinson. Octavius enters, visibly bereft over the death of Ramsden’s friend Mr. Whitefield, who was like a father to Octavius. They commiserate, and Ramsden temporarily lifts Octavius’s spirits when he mentions that Mr. Whitefield had wanted Octavius to marry his beautiful daughter, Ann. Octavius is infatuated with Ann but suspects he’s not good enough for her: Octavius, after all, is just a poet. Ramsden admits that there’s another obstacle standing in Octavius’s way: Ann is smitten with Octavius’s old school friend Jack Tanner, a garrulous libertine with radical anarchist views.
As if on cue, the maid returns and announces that Tanner is here to see Ramsden. Tanner enters and announces that he and Ramsden have a problem: in his will, Mr. Whitefield named Ramsden and Tanner as Ann’s co-guardians. Neither man is happy about this. They know Ann is trouble, and they don’t want to be held responsible when she inevitably does something awful and blames them for it. Tanner suggests they ask Ann which of them she wishes to be her guardian.
Ann and her mother, Mrs. Whitefield, join the men in Ramsden’s study, and they discuss the problem at hand. Ann, feigning deference, claims she can’t possibly go against her father’s wishes. Ramsden and the women leave, and Tanner and Octavius discuss Ann’s moral character and the roles of men and women in society. Tanner describes his philosophy that women lure men into marriage in order to use them to fulfill life’s higher purpose of procreation. That’s a problem for men, since in their role as breadwinners they have no time left for self-improvement or for creative endeavors. Tanner, the anarchist, is decisively against marriage; Octavius, the poet, defends love.
Anna and Ramsden return with the shocking news that Octavius’s sister Violet is pregnant while unmarried. Everyone thinks this is a huge scandal except for Tanner, who is pleased that Violet is fulfilling her higher purpose without cowing to the stifling social institution of marriage. The others think maybe Violet should go away for the duration of her pregnancy. They leave to go talk some sense into Violet, leaving Ann and Tanner alone in the study. Ann and Tanner reminisce about their childhood together. Tanner’s philosophizing bores Ann, and she tries to flirt with him. Tanner resists her.
The others return with Violet, who confirms that she is leaving immediately. Then, she shocks them with the news that she’s actually married, though she refuses to tell them her husband’s name. Violet’s secrecy scandalizes everyone further.
Outside the Whitefields’ house in London, Tanner and his chauffeur, Straker, are supposed to go for a drive with the others, but Tanner’s steam car has broken down. While Straker works on the car, Octavius admits to Tanner that he proposed to Ann the night before, but she claimed she was too preoccupied with grieving her father to marry.
Ann, Mrs. Whitefield, and Ramsden join the others outside, along with Hector Malone Jr., an American young man and the son of a wealthy businessman (Hector Malone Sr.). Hector is supposed to drive half the group in his car. Deviously, Ann asks Mrs. Whitefield for permission to join Tanner on a cross-country road trip—an offer Tanner made assuming that Mrs. Whitefield would say no, given her distrust of Tanner. Tanner is horrified when Mrs. Whitefield agrees, and Ann and Mrs. Whitefield run inside to pack Ann’s things.
The men talk among themselves. Hector mentions wanting to take Octavius’s sister Violet out for a drive, and things get awkward when the others have to tell him about the weird situation with Violet’s secret husband. Violet emerges from the house just then, and Hector requests to speak with her in private. The others oblige and leave them be.
Once the coast is clear, Violet and Hector kiss—Hector, it turns out, is Violet’s mystery husband. They argue over keeping the marriage a secret, but Violet insists on it. Hector’s father wants his son to marry a woman with a title and would disapprove of the marriage due to Violet’s middle-class background, and Violet doesn’t want Malone Sr. to cut off Hector’s allowance.
Tanner and Straker return just then, bringing the conversation to a halt. Violet and Hector go off together. Straker has long since repaired Tanner’s car, so Tanner—horrified at the prospect of a road trip with Ann—orders Straker to pack their bags. Then, Tanner and Straker speed off into the distance.
In the Sierra Mountains in Spain, a band of brigands has gathered near a mountain pass. Their leader, the anarchist Mendoza, is holding a lively debate about the relative merits of socialism and anarchism when the group spots an oncoming car. It’s Tanner and Straker. The group takes them hostage and demands a ransom for their release. Tanner is fine with this as he has lots of money. He and Straker make casual conversation with Mendoza around the campfire before drifting off to sleep.
The scene fades into Tanner’s dream, which takes place in Hell and features the play’s characters as figures from the Don Juan legend. Tanner is Don Juan, Mendoza is the Devil, Ann is Dona Ana, and Ramsden is the Statue (the reanimated form of Dona Ana’s deceased father). They all engage in a spirited debated in which Don Juan fleshes out his philosophy of life, namely his concept of the “Life Force,” the universal impulse that pulls humanity to improve itself through self-contemplation until it reaches its supreme form, the “Superman.” The Life Force complements—and is simultaneously at odds with—women’s higher calling to procreate, which compels women to entrap men in marriages that inhibit men’s capacity for contemplation and self-improvement. Ana argues with Don Juan about the moral implications of his rejection of marriage. The Devil argues with Don Juan about the latter’s insistence on purpose and direction: the Devil thinks people ought to just enjoy themselves. The Statue argues against Don Juan’s immorality and dismissal of love, though he ends up agreeing with Don Juan on some points, especially after admitting to his own moral hypocrisy.
In the end, Don Juan decides that he’s better suited for Heaven, which is dull but offers him the sense of purpose and direction that Hell lacks. He leaves Ana behind. When Ana asks where she can find Don Juan’s “Superman,” the Devil tells her he doesn’t exist yet. She declares her intention to find “a father for the Superman!” and then disappears.
The scene returns to the Sierra Nevada just as Tanner and Straker’s friends arrive, accompanied by soldiers, to rescue them. Tanner protects Mendoza against the soldiers, claiming Mendoza is his “escort” rather than his captor.
Everyone is now staying in a villa in Granada. Violet sends Straker a letter to deliver to Hector, but Straker messes up and delivers the letter to Hector Malone Sr. instead. Hector joins them, and an argument ensues between father and son. The others join them, and Hector finally admits to being Violet’s husband. As expected, Malone Sr. angrily cuts off Hector’s allowance, but he reneges after Tanner’s and Octavius’s offer to assist the couple wounds his pride. Hector is still fired up and insists on supporting himself without his father’s money. Alone with Malone Sr. later, Violet accepts the allowance on Hector’s behalf and agrees to talk some sense into her foolishly proud husband.
In a private moment with Ann, Octavius declares his love for her. This time, she denies him outright but lies and tells him it’s because Mrs. Whitefield wants her to marry Tanner. Octavius sobs alone in the garden, where Mrs. Whitefield later encounters him. Octavius tells Mrs. Whitefield what Ann said. Mrs. Whitefield realizes Ann’s lie and tries to convince Octavius that Ann lied, but he refuse to think poorly of Ann. Alone with Tanner later, Mrs. Whitefield officially gives him her permission to marry Ann. Tanner protests, but Mrs. Whitefield insists that he and Ann are actually the perfect match.
Tanner notes Mrs. Whitefield’s contempt for her daughter—and her paradoxical wish for Ann to marry—and sees his “Life Force” at work. When Ann returns for Tanner, he finally agrees to marry her. The others emerge from the villa, and Tanner grudgingly announces his and Ann’s upcoming nuptials.