The story starts with the basics: who, where, and what. We're in a small American town, meeting the Kellers and their neighbors. With the "whats" that start to accumulate – the ominous destruction of Larry's tree, the surprise appearance of Ann – we start to get a sense that Miller is creating a tense platform for some really big stuff to go down.
This is Big Stuff #1. Why isn't Mama Kate happy that her son wants to marry the girl next door? Because this girl next door was first engaged to Larry, the dead son. If Kate and Joe give their blessing, they're admitting that Larry's really dead. Chris's announcement begins to make the old conflicts come to the surface.
Big Event #2. The appearance of George tightens the vice grip on poor Joe Keller. George believes Joe is guilty, not only of shipping faulty parts but of pinning the crime on George's father. He doesn't want the marriage to go forward either.
This is the moment in the play when the audience stops breathing. Ann has given Chris a letter she received from Larry, a suicide note confessing that he knew about his father's crime and therefore won't go on living. The emotional peak of the play, this letter forces Joe to change his point of view. He admits – and understands – his guilt.
Understanding that both of his sons blame him for murder – and accepting the charge himself, at long last – Joe Keller goes into the house. What's he doing, we wonder. Taking a break from the trauma? Readying himself for jail?
We get our answer. Joe Keller has killed himself.
After the gunshot, a few lines follow to the end of the play. Kate begs Chris to not blame himself.