Lady Britomart and her son Stephen sit down to discuss the family's finances. Although Lady Brit seems to have plenty of confidence in her own opinions, she claims she wants Stephen's help working through a tricky issue.
Lady Brit's two daughters—Sarah and Barbara—are now engaged, and she is concerned about their ability to support themselves in their married lifestyles.
Stephen then gets taken into Lady Brit's confidence and learns a whole lot more than he ever wanted to know about the family history/finances.
It seems that his father, Andrew Undershaft, was a foundling adopted by the Undershaft family and groomed to take over its munitions businesses. The tradition of doing just that—leaving the family business to an adopted foundling rather than any of the family's biological children—had been going on for generations. So, Stephen would be skipped as well.
Apparently, this tradition was the reason Lady Brit and Andrew separated some years ago—she wasn't into the idea that her son would be disinherited.
Despite her issues with her husband, Lady Brit tries to push Stephen into "advising" her to take Andrew's money to make sure the girls have a suitable living once they're married.
However, having heard all these new details of his father's past/behavior, Stephen is not inclined to be involved with his father at all. However, Lady Brit basically bullies him into making that his "advice."
Which is a good thing—since Lady Brit has already invited Andrew over to ask him about it. This will be the first time the kids have seen their father in a long time.
Then, Sarah and Barbara enter with their fiancés, Charles and Adolphus, and hear that their father is on his way to see them.
When Andrew arrives, there's a great deal of hilarity as he attempts to figure out who is who—it seems he doesn't actually remember how many kids he has . . .
Eventually, formal introductions happen and he gets his own offspring straight in his mind.
They then all sit down to chat and learn more about each other. This leads into a discussion of everyone's different views on religion and morality—and Andrew's profession as a weapons manufacturer/dealer.
During all this chatter, Barbara and her father make a deal: Her father will visit the Salvation Army, where she works, if she visits his munitions factory.
Lady Brit gets annoyed that they keep talking about religion, so she decides to do it "properly" by holding formal prayers. At this point, the rest (minus Stephen) excuse themselves to do a different kind of service in the drawing room. Lady Brit is annoyed at being abandoned, but even she eventually gets up and joins them. Stephen refuses.