There are so many reasons why The Unbearable Lightness of Being doesn't follow a typical plotline. To begin with, the novel features several different, interwoven, plotlines revolving around several different protagonists. Even if you identify a climactic situation for one character in one plotline, it doesn't necessarily serve as a climax for the plot as a whole. Additionally, the same events are narrated more than once from different characters' points of view. The plotline is not only non-chronological, but also non-linear. Lastly, the novel is as much a philosophical work of ideas as it is a fictional story of characters, which means we can't break it into purely plot-driven stages.