Before we start, you should know that The Unbearable Lightness of Being covers several intersecting plotlines and is told in a non-chronological, non-linear fashion. Additionally, there are frequent breaks in the plot in which the narrator discusses the novel's philosophical themes. We'll try to get the general gist across to you in this brief summary, but there's really no substitute for reading this remarkable and incredibly interesting novel.
The novel begins with a discussion of Friedrich Nietzsche's idea of eternal return. Nietzsche theorized that everything we experience happens an infinite number of times. But if the opposite is true, argues the narrator of The Unbearable Lightness of Being, and if our lives happen only once, then they aren't that significant. They are meaningless and carry no weight. The narrator also considers the question of which is better – lightness or weight? Which do we want?
Now we launch into the story, which takes place in Prague in the late 1960s. We meet Tomas, a 40-year-old doctor who lives in Prague. Tomas is an epic womanizer who is with a new woman every day. He meets, falls in love with, and marries a young woman named Tereza, but doesn't stop sleeping around. He feels that sex and love are two different things, and that he is emotionally faithful to his wife. Tereza is entirely dependent on Tomas; she hates his womanizing but considers herself too weak to leave him. She forms a close relationship with the female dog Tomas buys her, whom she names Karenin (ironically, a male name) after a character in Anna Karenina.
When the Russian army invades Prague in 1968, Tomas and Tereza leave for Zurich. Shortly after the move, however, Tereza breaks down at the thought that Tomas is still cheating on her even in a new city. She returns to Prague. Tomas decides to follow her home, even though Prague is in political turmoil. The narrator breaks from the story to discuss the notion of "es muss sein!," which is taken from a Beethoven motif and translates to "It must be!" Tomas believes that Tereza is a part of his es muss sein, and that he has to be with her. Shortly after returning to Prague, however, he realizes that she is in his life by mere chance. Tomas decides that Tereza was born of fortuity (of a lucky event), and not of some kind of destiny or es muss sein.
Another plotline revolves around Sabina, a painter and one of Tomas's many mistresses. Sabina's sexual prowess rivals that of Tomas. We learn early on that she is a sexual force to be reckoned with. After the Russian invasion, Sabina immigrates to Geneva and takes a lover named Franz, a professor who is married to a not-so-likeable woman. Sabina and Franz's relationship is characterized by hopeless misunderstandings and miscommunication, yet they are happy. Eventually, Franz decides he only wants Sabina and tells his wife about the affair. Sabina quickly leaves Franz, but he ends up happy without her. It turns out that he liked the idea of Sabina more than Sabina herself, so he can worship her from afar now that she is gone. He gets into a serious relationship with a student of his.
The narrator then pauses to explore Sabina's actions. He reasons that her whole life has been a string of betrayals. He wonders if perhaps she is pursuing lightness through the act of betrayal. The narrative then jumps years ahead, to a time when Sabina is living alone in Paris. She learns of the death of Tomas and Tereza and mourns their passing.
The narrative jumps back in time to Tereza and Tomas in Prague after their return from Zurich. Tomas writes an anti-communist article and loses his job at the hospital as a result. He ends up a window washer for two years. Tereza ends up working at a bar. Tomas's womanizing gets worse than ever, and Tereza is haunted by it day and night. At one point, she tries to have meaningless sex with another man to live as Tomas does, but is unable to enjoy it. The narrator explorers Tereza's childhood and the consequences that her relationship with her mother have on her body image. In particular, Tereza resents her body and believes that her "I" (her sense of herself) rests in her hidden soul.
We see the two years in Prague a second time, now with a focus on Tomas's experiences. We learn more about Tomas's refusal to retract his anti-political article, as well as the rationale behind his lifestyle. The narrator claims that both medicine and womanizing are part of Tomas's es muss sein, whereas Tereza was his voluntary choice. Some time after he loses his job, Tomas's adult son from his first marriage contacts him. They have never met before, since Tomas cut off contact with his first wife after their divorce. The son is a political rebel and wants to enlist Tomas's help. After some deliberation, Tomas decides to decline amiably. As time passes, Tomas is aware of the toll his womanizing is taking on Tereza, and he's had just about enough of washing windows. The two of them decide to move to the countryside.
The narrator breaks from the narrative to discuss kitsch, a German term for an aesthetic ideal that denies the existence of the ugly side of life. (The word is German in origin, but has become a part of the American vernacular as well.) We jump for a moment to Sabina ten years later, living in America. Sabina hates communism, the narrator explains, because it is, like all political platforms, a form of kitsch.
Then we jump to Franz and his student-mistress some years after Sabina left him. He is invited to a political protest march, and decides to go because he feels Sabina would have wanted him to. While there, he is mugged and ends up paralyzed in a hospital, unable to move or speak. His old wife, Marie-Claude, takes care of him, though Franz hates her and wants desperately to see his student-lover. When he dies, Marie-Claude makes it seem as though Franz simply had a mid-life crisis, but loved her all along.
We learn that Tomas and Tereza died when they were crushed in a car accident. Tomas's son, who is now given the name Simon, takes care of the funeral. Because Simon has become religious, he puts a religious phrase on Tomas's gravestone, though Tomas never would have wanted that. We learn that Simon writes letters to Sabina, as she is the only person that he knew was close to his father. In America, Sabina writes into her will that she wants to be cremated and scattered to the winds after she dies. Interestingly, she recognizes that Tereza and Tomas died under weight, and she wants to die under lightness.
The narrative again goes back in time to when Tereza and Tomas move to the country. They both work for the community farm; Tomas drives a pick-up truck and Tereza herds cows. The relationship between Tereza and her dog Karenin is explored in great depth, with much philosophizing on the soul of an animal, the nature of happiness, and the passing of time. Karenin ends up getting cancer and is eventually put down. The novel ends when Tereza and Tomas go out dancing with some friends and retire to a hotel room for the night.