The group leaves Barbara and Cusins alone. Cusins confesses that he does not regret the sale of his soul as he has sold it not for money, position, or comfort but "for reality and for power." For Cusins, all power is spiritual and both good and bad. He wants to "make power for the world," to create what teaching a dead language and civilization could not. He yearns to arm the commoner against the intellectual and not vice versa. He will provide a power "simple enough for common men to use yet strong enough to force the intellectual oligarchy to use its genius for the general good." As the gun can destroy all the higher powers, man must master it first.
Barbara pledges to stay with Cusins. Indeed, if Cusins had not accepted Undershaft's offer, she would have married the man who accepted it. She sees the souls saved by her father's community. She has renounced the Army's bribes of bread and heaven. She will return to the Army, but now God's work will be done for its own sake.
Cusins flourishes his imaginary drumsticks in joy. Angered by his levity, Barbara yearns to flee to heaven. She was happy in the Salvation Army until she realized Bodger and Undershaft provided for and saved its guests. Barbara wishes she could cure Cusins of his middle-class morality. As the daughter of a foundling, she comes "straight out of the heart of the whole people" and thus has no class. Cusins asks if the "way of life" then lies through the "factory of death." Barbara answers: "Yes, through the raising of hell to heaven and of man to God, through the unveiling of an eternal light in the Valley of The Shadow."
The two embrace and Barbara runs to the shed and calls, childlike: "Mamma! Mamma! I want Mamma!" Britomartemerges and chides her daughter as she clutches at her skirt. They leave to choose a house in the village. Undershaft reminds Cusins to come at six o'clock next morning.
This reconciliation between the two lovers functions as an epilogue of sorts to Undershaft's speech that completes their conversion to his dogma. In the preceding scene, Barbara in particular experiences what she describes as an "earth-shaking" epiphany, realizing that the Undershafts and Bodgers of the world, rather than God, hold her in their power. Whereas her mother sees houses and gardens in the company town, Barbara now recognizes that these millionaires bear the means of salvation. Turning one's back on such men, as Lady Britomart attempts to do in the preceding scene, is a turning from what she rather grandly terms "life" itself. Barbara will return to the Army with Undershaft's gospel on the crime of poverty, the imperative to "kill" this social ill, and the sacraments of money and gunpowder.
Similarly, Cusins does figure his decision to join the Undershaft firm as a turn to life. Cusins has sold his soul for "reality and power," a reality and power attained through the exercise of his will on behalf of his beloved people. Through the armory, he will abandon his anachronistic and intellectualizing studies and make power for the contemporary world, a power accessible to the masses and that forces the "intellectual oligarchy" to exert itself for the general good. Like Undershaft, Cusins comes to apotheosize the arm as the power than can destroy all others and determine the course of the world.