The Unbearable Lightness of Being opens with a philosophical discussion of lightness versus heaviness. Kundera contrasts Nietzsche's philosophy of eternal return, or of heaviness, with Parmenides's understanding of life as light. Kundera wonders if any meaning or weight can be attributed to life, since there is no eternal return: if man only has the opportunity to try one path, to make one decision, he cannot return to take a different path, and then compare the two lives. Without the ability to co mpare lives, Kundera argues, we cannot find meaning; where meaning should exist we find only an unbearable weightlessness. The uncertain existence of meaning, and the opposition of lightness and heaviness, the key dichotomy of The Unbearable Lightness of Being, sets the stage for the entire novel.

The year is 1968. The protagonist, Tomas, a brilliant Prague surgeon, pursues a philosophy of lightness in his erotic adventures and exploits. Briefly married in the past, he neither sees nor wishes to see his ex-wife or young son and is comfortably established as a perpetual bachelor. He meets Tereza, a café waitress in a town he visits, and realizes when she follows him to Prague that she intends to "offer him up her life." A determined libertine, he momentarily resists his budding roma ntic feelings for her, then gives in to his love.

Tereza had been living a frustrated life as a waitress in a small town, and dreamed of escaping, especially from her vulgar mother. She recognizes in Tomas an intellectual and dreamer, and falls in love with him instantly. The two live together, but Tomas is unable to give up his mistresses. For a while he hides his infidelity from Teresa. Eventually he admits to it, but claims that his sexuality is entirely separate from his love for her. Tereza, unable to accept his behavior or adopt a light attitud e towards sex, suffers increasingly from nightmares, and contemplates suicide.

To keep Tereza happy, Tomas marries her. He keeps his mistresses, however, including his closest friend and long-term lover Sabina, a beautiful, reckless, and talented painter. In spite of herself, Tereza is charmed by Sabina's openness and light-heartedness, and the two women grow friendly. Sabina finds Tereza a job in Prague as a photographer. Despite her friendship with Sabina, however, Tereza's jealousy of Tomas does not slacken.

The events of the Prague Spring result in the Soviet military occupation of the city. Tomas, who in the past wrote an article condemning the Czech Communists, is warned to leave. Sabina flees first, and later Tomas and Tereza join her in Switzerland. Tereza, who found some fulfillment in her job as photographer in Prague, realizes that in Zurich she is jobless and must sit at home while Tomas continues having affairs. She decides that "when the strong were too weak to hurt the weak, the weak had to be strong enough to leave," and returns to Prague. Tomas attempts to enjoy his newly recovered freedom for a few days, then gives up and returns to Prague and Tereza. The return truly means giving up freedom—there is no chance that the couple will be allo wed to leave again. In Prague, Tomas's political troubles escalate. He loses his position as surgeon for refusing to sign a denunciation of his anti-Communist article. Both the Communist regime and underground dissidents attempt to seduce him to their side. His own son reappears as a young dissident and preaches to Tomas with no success, for Tomas hates the idea of being used politically in the same way Sabina hates artistic kitsch. In the end, Tomas seeks obscurity in a job washing windows. His fame persists, however, and he continues seducing the women he works for.

Tereza, now a bartender, in a moment of desperation has an affair with a tall engineer who comes to her bar. She does so in hopes of coming closer to Tomas's way of life; instead she grows more miserable and becomes convinced the man was a police agent hired to gather potential blackmailing material. After many scenes and nightmares, she convinces Tomas to move with her to the country. This means giving up their way of life entirely, and an end to Tomas's erotic adventures.

After living peacefully in the country for some time, Tomas and Tereza are killed one night in a driving accident; they die instantly and together.

In Geneva, Sabina has a love affair with Franz, a university professor and idealistic intellectual who has more in common emotionally with Tereza than Sabina—he imbues his life with heavy meaning. He views Sabina as a romantic and courageous Czech dissident, and is tortured that he must betray his wife Marie-Claude in order to see her. Sabina loves Franz but their views on betrayal differ dramatically; whereas he hates the idea of betrayal, she views betrayal as the first step towards "going off into the unknown," the most glorious thing she can think of.

When Franz leaves his wife and expects to move in with her, Sabina abruptly leaves Switzerland. Sabina leaves Geneva for Paris and then Paris for America; she learns of Tomas and Tereza's deaths from a letter and understands her last link to the past has been broken. She ends up living with an elderly American couple and wondering if she has reached the end of her perpetual flight.

Franz remains separated from his wife. After Sabina's betrayal, he finds consolation with a young student, a girl in over-sized glasses who loves him simply. Franz never accepts that he clearly misunderstood Sabina, but continues to hold her image as in ideal in his head, (wrongly) thinking his decisions in life would have made her proud. At his death, his wife reclaims Franz's body and orders the words "A return after long wanderings" written on his tombstone.