The play begins in the household of Lady Britomart. She discusses with her son, Stephen how to supply her daughter Sarah, who is engaged to Charles Lomax, with enough annual salary to get them by until Charles comes into his family's money when he turns 35. She also discusses the same topic for her second daughter, Barbara who is a Major at the Salvation Army and is engaged to Adolphus Cusins, a scholar of Greek philosophy.
Lady Britomart convinces Stephen that the only way to do this is to ask her estranged husband, and father to all three children to supply the money. He left the family early in the children’s lives and is a manufacturer and supplier of weapons of all kinds. He is one of the wealthiest men on earth and makes his fortune by designing ways to kill men. Stephen is sickened by this, but he doesn’t know that his mother has already invited his father, Undershaft to the home and he will be there any minute.
Undershaft arrives and Sarah, Lax, Barbara, Cusins and Stephen all speak to him. Undershaft takes to Barbara, even agreeing to come to the West Ham location of the Salvation Army to see what she does there. In return, he asks her to come to his factory to see his work. Barbara is I. The business of saving souls, her father the business of killing men.
At the Salvation Army we see characters of the lower class appear and speak about how they use the Salvation Army for a free meal. Some are sincere in their giving their life to God, while others say it but continue to live by stealing—they only say what’s necessary for a meal. We see Barabara command a man, Bill, who has come in and punched two women in the face, I. Such a way that makes him on the verge of giving his soul to God. That is until she rejects Bill’s money, offered as a way to be even for striking the women, and the Salvation accepts Undershaft’s money knowing he makes millions killing people. Thus, the Salvation accepts the money of a wealthy sinner but not a poor one. This turns Bill against Barbara and he leaves unsaved.
After Undershaft’s money is taken by the Salvation, Barbara leaves her post. She’s lost faith in what they do. Later, Undershaft arrives at Lady Britomart’s Home to take everyone to his factory. She demands he give his inheritance to their son Stephen, but the tradition of inheriting the Undershaft fortune is that it must be given to a foundling to be trained in running it, not a direct heir.
Stephen says he wouldn’t take it anyway, and at the factory Adolphus reveals he is a foundling. With this newly exposed, Undershaft offers the inheritance to him. It is against his morality to accept, but he does anyway. Stating that he desires to arm the Conan man against the intellectuals as he has lived his life to this point arming the intellectuals against the common man.
His acceptance causes Barbara to agree to marry him rather than leave him. She stayed that she can save the souls of the fortunate men and women who work in the factory, and reveals if Cusins would have declined the offer she would have left him and married the man her father gave the inheritance to.