You might be wondering: if there are four parts to this book, how can it be divided into three acts? Fair question. Let's start with how Steinbeck breaks up the novel:
Part One: The beginning of the Trasks to Cathy marrying Adam and sleeping with Charles. In other words, Part One ends when the action moves west to California. Let's call this the First Generation part.
Part Two: From when Adam and Cathy come to California through to when Adam finally gets his father-act together and names the twins. Part Two ends with Adam's recognition of his sons, so it's the Adam Has to Face the Facts part… which is the best name we could come up with.
Part Three: Part Three is really about the Hamiltons and their saga, so it's the Hamilton part, and it ends with Tom's suicide. Part Three is also where we get to see Cathy become Kate and watch as she manipulates her way to the top of the whorehouse food chain.
Part Four: This part begins with Adam, Lee, and the boys moving to Salinas and lasts until the end, so all the focus shifts to them. In other words, Part Four is the Cal and Aron part.
The way we've done it, we take Cathy shooting Adam as a pivotal moment that serves as the climax for the first act, because really, it's a big deal for the plot. Then we've smushed Adam's recovery/realization period together with all the Hamilton drama, because it's a bit of a break from the Cain-Abel plot arc. And for the final act, Cal and Aron are definitely the stars of the show.
So, Shmoopers, here it is:
In Act I we meet the first generation of Trasks and see how the whole Cain-Abel thing is working out for them (spoiler: it's not). Cathy's shooting Adam is the game-changer that brings the first act to a close.
Act II could be called What To Do When Things Don't Work Out the Way You Planned. This act is actually more heavily focused on the Hamiltons than it is the Trasks—they have plenty of their own family issues to deal with.
In Act III Cal and Aron grow up and become the new main characters of the story in Part Four. This act would probably be called The Trasks: The Sequel.