The event in which Biff discovers the truth about his father, Willy, in the play Death of a Salesman, by Arthur Miller, occurs during Biff's last year of high school when he is on the brink of being accepted to college.
The event comes in a flashback in Willy's mind. That Summer, Biff receives news that his Math teacher, Mr Birnbaum, will not pass him in Math. This would mean that Biff not only will not graduate high school, but that his college football aspirations will never occur.
Frustrated, Biff takes off to Boston to meet his father since Willy was his biggest support system. When he goes to see his father at his hotel room, Willy is obviously not expecting him. After Biff and Willy outside Willy's hotel room about a way to convince the Math teacher to pass Biff, father and son are having a jovial conversation. This is until Biff hears a woman in the bathroom as if taking a shower in Willy's room.
Willy tells Biff the lame excuse that they are painting the woman's own room, and that she needed a shower, so she went to Willy's room to take it there. The woman comes out, either drunk or clueless, joking around with Willy, saying that there is something in the bathtub. Willy treats the woman as she is a stranger.
The woman becomes beligerant when Willy pushes her out and she demands that he gives her the stockings that he promised her. All this is witnessed by Biff, who is astonished and particularly hurt by the stockings issue: After all, his own mother slaves over her only pair of stockings while his father gives them new to other women.
When the woman gets her box of stockings she actually addresses Biff asking him what he plays. She answers "that's me too" when Biff responds that he "was football", and she leaves the scene.
Biff becomes motionless, cries in place, and does not move. His feeling of deception is evident in that, when his father tries to deflect the event by switching the topic towards convincing the Math teacher, Biff answers that the teacher would not listen to Willy. This is what demonstrates that Biff cracked the fantasy that Willy created for his family: That he was a successful businessman and a successful family man. In Biff's eyes, Willy is now powerless, weak, and worthless. It is the end of his father, as he knows him.