Undershaft returns to the subject of inheritance and tries to tempt Cusins by quoting Plato to him. He asks if he will see Cusins at 6 am the following day (to start work) and Cusins says firmly his hours are from 11 am to 5 pm. Undershaft agrees to this and says before a week has passed he will be coming in at 6 and stay until he is turned out. He then encourages the others to look in the explosives shed to allow Cusins and Barbara to talk. Before leaving, Stephen tells Cusins this business ‘is one of the highest character and a credit to our country’. He adds that he is proud of his father.
When they are alone, Cusins tells Barbara he will accept Undershaft’s offer and had to decide without consulting her so she did not have the burden of choice. He says he wants to ‘make power for the world’. She says this is what she also wants, but it must be a spiritual power. He argues that all power is spiritual and reminds her that cannons do not go off by themselves. He also believes, ‘you cannot have power for good without having power for evil too’. He wants to give the common man weapons against the intellectual man.
Cusins explains how, when the Turks and Greeks were last at war, his best pupil left to fight for Greece. Cusins gave him a revolver and 100 Undershaft cartridges (rather than Plato’s Republic). He thinks that any blood shed by this man is on his head as well as Undershaft’s and ‘that act committed me to this place for ever’. He dares to make war on war now and asks if their relationship is over.
She says not, but warns him not to be light about this. In the Salvation Army, she escaped from the world for a moment ‘into a paradise of enthusiasm and prayer and soul saving’. This lasted until the money ran short. She thinks there is no escaping the influence of Bodger, her father and other fellow capitalists: ‘Turning our backs on Bodger and Undershaft is turning our backs on life.’ As the daughter of a foundling, she believes she has no class and so will not turn her back on his business (as the middle classes would). She tells him if he had turned her father’s offer down, she would have given him up and married the man who accepted it. She sees all the human souls there are to be saved and this is where salvation is really wanted. She adds that her courage has not deserted her and Major Barbara ‘will die with the colors’. The play ends with Undershaft saying, ‘six o’clock tomorrow morning, Euripedes’.
In this final part of the play, it is perhaps surprising that Cusins accepts Undershaft’s offer to take over the cannon business as his foundling heir. The reasons for this are made more apparent when Barbara argues that, ‘turning our backs on Bodger and Undershaft is turning our backs on life’. Both Cusins and Barbara come to an agreement that it is impossible to avoid the influence of capitalism, but it is perhaps a strange and unlikely outcome for a play that has eschewed the power of the capitalist up to Act III. Their acceptance may be interpreted as highlighting the pervasive influence of this dominant ideology, but, conversely, it may be read as a choice to engage in society rather than escape from it through just the charitable work of the Salvation Army.