The setting of this scene is a platform on a hill overlooking the model village of Perivale St. Andrews, and the area is surrounded with "several dummy soldiers more or less mutilated." Major Barbara — now merely "Barbara," as in the preceding scene — is seen alone, observing the entire valley. Cusins enters, admiring the perfection of the place, and he informs Barbara that they have indeed found employment for Peter Shirley but that the man is frightfully unhappy because the job involves "mental time-keeping" and he is uncomfortable in the refined lodgings that they have provided for him.
Next, Stephen arrives filled with enthusiasm for the place. Cusins agrees with him and says that everything is "horribly, frightfully, immorally, unanswerably perfect." Then Sarah arrives exclaiming about the magnificent facilities that are available. Then Andrew Undershaft enters with the good news that the new Lazarus and Undershaft battleship is a huge success: "It wiped out a fort with three hundred soldiers in it" in the Manchurian War, but, as yet, he doesn't know which side mastered this victory. Stephen compliments his father on the "wonderful forethought, the power of organization, the administrative capacity, the financial genius, the colossal capital" that the community represents, but he wonders if things aren't too perfect. Undershaft responds by pointing out that there is a certain built-in anxiety because "we may be blown to smithereens at any moment."
Bilton, the foreman, arrives with Charles Lomax from inside the high explosives shelter where Lomax had carelessly lit a cigarette and dropped the lighted match on the floor. Lomax ignores the seriousness of his actions and thinks that the precautions are "a bit of tosh."
Lady Britomart arrives then, and as fits her concerns, she is terribly impressed with all the domestic arrangements, but she is unhappy at the thought that all of this magnificence might pass into someone else's hands. She even goes so far as to suggest that Cusins be made the successor, and Undershaft is quite willing because Cusins is the exact type whom he is looking for, but, unfortunately, Cusins is not a foundling. By a clever ruse, then, Cusins points out that this objection could be overcome because, technically, while his parents' marriage is legal in Australia — in England, "My mother is my father's deceased wife's sister and in this island I am consequently a foundling." By this subterfuge, Undershaft acknowledges that Cusins is indeed eligible to be an heir. However, Cusins is not so easily persuaded to take the job; he says, "There is an abyss of moral horror between me and your accursed aerial battleships."
In spite of all the differences, however, it is agreed that Cusins will take over if a proper bargain can be struck, with Cusins demanding much more money and a share of the profits, and Undershaft reluctantly agreeing — if Cusins agrees to the basic code of the Armorer — that is, Cusins must sell weapons to all people, regardless of the principles of the men who are buying the weapons, and, furthermore, Cusins must be willing to change his name (Adolphus Cusins) to Andrew Undershaft.
Cusins has no difficulty deciding whether or not to change his name, but he does have difficulty with the moral question. Undershaft maintains, however, that one can make weapons, but one cannot make men of courage and conviction. He then turns to Barbara and addresses her: It is easy, he says, to convert people's souls by promising them bread and treacle to eat. Poverty will force people to agree to anything. In fact, poverty is the worst of all crimes. He then challenges her to scrap her old ideas in the same way that he would scrap a bad plan or a bad idea. Come to this model town, he says, where everything is cleanliness and prosperity and test her theories.
Undershaft further maintains that since poverty is the most heinous of crimes, destroying more men and affecting society in a worse way than do wars or pestilence, more than murders and robbers, then it must be done away with. When Barbara is reluctant to leave her wards in London's East End, Undershaft reminds her that he himself was once a member of the poverty stricken East End, and he can assure her that there is nothing good about poverty; let her bring her Bill Walker with her, and she can save him from poverty — then, when he is comfortable physically, Barbara can more firmly meet the challenge of saving his soul.
When Undershaft turns to Cusins for a decision, Cusins maintains that he wants to "avoid being a rascal." Undershaft rejects such appeals to love, pity, and personal righteousness as being of no social value; if you want to change society, he says in effect, you have to have a large amount of money to do so. When Cusins ultimately maintains that he "hates war," then Undershaft challenges him to take a position so as to become powerful enough to "make war on war" — in other words, take the position as chief of the munitions factory.
Undershaft leaves then to show the others the inside of one of the munitions shelters.
Left alone, Cusins tells Barbara that he is going to accept the offer. When Barbara mentions Cusins' having "sold his soul," Cusins tells her that he has sold his soul for lesser things — for taxes, for professorships, for income, and so forth, and now he will sell it "for reality and power" — not for himself, but for the world. He emphasizes that cannons do not kill people by exploding themselves; people must set them off, and he now realizes that one "cannot have power for good without having power for evil too." He wants to use power to bring about a sense of equality among people — that is, he wants to give the common person a weapon to use against tyranny of any type.
Suddenly, Barbara agrees with him that he has made the right decision. She also realizes that her work with the Salvation Army was misdirected: "Turning our backs on Bodger and Undershaft is turning our backs on life." She further realizes that the wicked and evil in the world are also a part of the totality of life. She now sees that trying to save people's souls by giving them "a scrap of bread and treacle" in return for which there has been their "sobbing with gratitude" is no longer an acceptable approach. She maintains: "My father shall never throw it in my teeth again that my converts were bribed with bread." Now she will convert only well-fed men and women, and with her faith once again restored, Major Barbara returns to the colors.
This scene is set, paradoxically, at a place where the peaceful valley of Perivale St. Andrews is viewed from a perspective where it is surrounded by the symbols of death and destruction. The lovely perfection of the model town is seen against an immediate foreground of cannons and dummy, straw soldiers which have been theoretically destroyed by the instruments of destruction created by Andrew Undershaft. Furthermore, all of the characters are standing next to one of the explosive shafts, and they can thus be blown up at any minute. Consequently, the physical surroundings comment upon the philosophical discussion — the contrast is as great as between Lady Britomart's insistence upon formalized rituals and Undershaft's insistence upon the right of many to destroy all types of rituals and to establish new governments and new religions. Thus, all of the discussions concerning peace, brotherhood, religion, and social security take place, paradoxically, amidst the instruments of destruction and the fake bodies of mutilated soldiers; furthermore, everyone is standing upon a hill which could at any moment explode and shatter each of them in infinitesimal pieces.
At the beginning of the scene, Shaw dramatically has each of the characters enter and admire various aspects of the model city. Since each character had expected to see some fiery pit of hell, a pit of horrible filth and stench filled with degenerate and filthy people, they are constantly surprised by the perfection of this place. This is Shaw's statement concerning the socialistic state. That is, only the rich can afford to establish the perfect socialistic state which in St. Perivale is also seen as the ideal utopian settlement, as attested to by the various reactions of each character. Stephen is impressed with the schools and the libraries, and he acknowledges that the entire place is a "perfect triumph of modern industry." He acknowledges to his father that the place represents "wonderful forethought, power of organization, administrative capacity [and] financial genius." Lady Britomart is overwhelmed by all of the domestic arrangements, and Barbara and Cusins are awed by the entire concept.
Ultimately, this final scene serves as the vehicle for Shaw's main ideas. First, Shaw (through Undershaft) reemphasizes the idea that poverty is the worst of all evils. Undershaft was brought up in London's East End amidst the worst types of poverty, and he assures his listeners that there is nothing romantic, charming, or attractive about poverty. Instead, he says, poverty is the most destructive thing known to man, rendering man a victim of filth, disease, and crime. Thus, Undershaft is the ultimate realist, and yet he is also a quasi-mystic. He strongly feels that bodies are to be saved first and then their spiritual needs tended to. This is the challenge which he confronts Major Barbara with; he challenges her to bring the Bill Walkers of the world to St. Perivale and give them physical security (freedom from want and hunger), and then, once the masses feel materially secure, Major Barbara can work to save their souls.
To change the world, Undershaft maintains that a person of great wealth, power, and imagination is needed. Cusins could be that person. As for Cusins, he is strongly attracted to Undershaft's argument to use his power to "make war on war." Thus, ultimately, Cusins will accept Undershaft's challenge, and Major Barbara will also acknowledge the value of saving the body first and then wrestling with the soul.