The end of Book 1, of course, when Eustacia learns that Clym is coming home from Paris, which basically sets everything else into motion.
In the midst of falling in love, these two crazy kids manage to seriously anger Mrs. Yeobright (hence the conflict). This love affair is far from calm, and we have the ongoing conflict with Damon as well.
Eustacia and Clym's marriage gets hurt by one problem after another, so the two grow distant. Damon and Thomasin aren't having a great time either. Mrs. Yeobright tries to give her relatives their inheritance, which of course goes badly.
Oh, the drama. This is the major turning point of the novel and Mrs. Yeobright's death sets the stage for all that follows.
The suspense is out of control here, given Eustacia's crushing guilty secret and Clym's obsessive need to find out about his mother's death.
It's all downhill in this section, though not in terms of action – everyone is pretty much just doomed. The requisite tragic deaths occur here.
Everything wraps up here and the (surviving) characters get their endings, though not all are particularly happy.