Part 1: Lightness and Weight

The novel opens with a meditation on philosopher Friedrich Nietzsche's idea of the eternal return, contrasted with the notion of einmal ist keinmal; that is, "what happens but once . . . might as well not have happened at all." According to Nietzsche, eternal return is the "heaviest of burdens." The absence of this burden, however, renders life inconsequential. The binary opposition of weight and lightness continues throughout the book.

Kundera next introduces Tomas, a surgeon who has fallen in love with a young woman named Tereza. Tomas has many mistresses, engaging in what he terms "erotic friendships." When Tereza discovers Tomas's many mistresses, she is distraught. It is this contrast between the weight of Tereza's love and the lightness of Tomas's love that provides much of the material for the book.

Eventually Tomas marries Tereza. He also buys Tereza a puppy they name Karenin. Although married...