Meet the Trasks, Part I
In order for us to get the whole Adam-Eve/Cain-Abel allegory, we need to set the stage, introduce the players, and show what kind of baggage they're carrying. For that, we've got the first generation of Trasks. Things seem to be going moderately well, aside from a few road bumps here and there, until Cathy shoots Adam and blows the whole illusion of Eden to smithereens.
Trouble in Paradise, Literally
Now it's up to Adam, Lee, and Samuel to make sure that the next generation of Trasks don't inherit the issues of their parents. The only problem is that it seems inevitable that they will. Cal is essentially a clone of Charles and Aron is making all the same mistakes that Adam did. What's worse, Cathy/Kate has gained quite the reputation for herself in Salinas, and it's only a matter of time before the boys find out. Talk about trouble brewing.
That Awkward Moment Where You Find Out the Truth
It all comes to a head when Cal gives his father the money he earned and is rebuked. That one act sets into motion all the tragedies of the novel: Cal showing Aron Kate, Aron joining the army, Adam having a stroke, and Kate's suicide. All over some beans (no, really).
Learning to Love Yourself While Also Being Wracked with Guilt
We have some things happen in the interim before the novel's final blow, like Cal and Abra developing a relationship, and Lee giving Cal some tough love advice about getting over himself. It gives us a sense that even though some really bad stuff may have happened, it isn't completely hopeless.
Father Forgives, Kind of
After Aron's death and Adam's severe stroke, we need to know that this story isn't just going to repeat itself in all subsequent Trask generations. So it's definitely a break-the-cycle moment at the end when Adam gives his blessing and essentially tells Cal that he has the choice to be whatever kind of person he wants to be.