The next day at lunch Lady Britomart appears writing in her library. Sarah sits reading near the window; Barbara, in ordinary fashionable dress, broods on the settee. Today the family plans to visit Undershaft's cannon works.

Lomax enters and bluntly expresses his surprise at Barbara being out of uniform. Though sympathetic to her disillusionment, he always knew that, unlike the Church of England, there was a "certain amount of tosh" to the Salvation Army. Britomart chastises him.

A weary Cusins enters—to the shock of the family, he was up late drinking with Undershaft after the meeting. Apparently Undershaft completed the "wreck of [his] moral basis, the rout of [his] convictions, the purchase of [his] soul."

Morrison announces Undershaft's arrival. Britomart sends the children out to ready for the excursion. Undershaft enters and before he has time to breathe, Britomart raises the topic of Stephen. Undershaft cannot disinherit him. Undershaft protests that tradition demands Stephen's dispossession. The firm needs a man without relations or education, a man who would be out of the running if he were not strong. The tragedy is that these days all the foundlings are snapped up by the welfare state, domesticated and crammed with secondhand ideas. In any case, if Britomart wants the foundry in the family, she should find a foundling to marry Barbara.

Stephen enters and announces that he wants no part of the foundry. When Britomart attempts to intercede, Stephen informs her that all discussion of his future will now take place with his father, between one man and another. A teary Britomart complies.

Chiding his son for so sulkily repudiating the foundry, Undershaft asks Stephen about his interests. Philosophy, the arts, and law are ridiculous to him. All Stephen knows above all, however, is the difference between right and wrong. Undershaft jeers and asks how Stephen can claim to know what has puzzled philosophers, artists, and lawyers through the ages. Like the Salvationists, respectable people are far too convinced they can teach morality. As Stephen knows nothing and thinks he knows everything, he should go into politics.