Barbara comes into the house and finds Billy in the rumpus room. She's carrying a copy of his letter to the Ilium newspaper about Tralfamadore, and she's freaking out.
Barbara's 21 and kind of mean because she's had too much responsibility too soon: she had to arrange her mother's funeral and she's worried that Billy has gone senile thanks to his plane crash.
Billy is only 46 and wants to persuade Barbara that he's not senile, but he doesn't have time to think of his booming optometry business because he has a higher purpose.
That higher purpose is to show mankind the wisdom of his "little green friends" (2.9.4) on Tralfamadore.