Most good stories start with a fundamental list of ingredients: the initial situation, conflict, complication, climax, suspense, denouement, and conclusion. Great writers sometimes shake up the recipe and add some spice.
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.