Kundera immediately poses the key paradox of The Unbearable Lightness of Being, the paradox of lightness versus weight. He asks what follows from the assumption that man may try only one path. If man cannot try different paths, and weigh them again st one another, does that mean that human life is characterized by unbearable lightness or meaninglessness? Is lightness splendid and weight a burden, or does meaning only come from weight? Against the background of these questions, the narrator begins th e story of Tomas, a surgeon living in Prague.
Tomas spends an hour with the waitress Tereza after meeting her in a small town café. Ten days later, she visits him in Prague. While there, she falls dangerously ill; after spending a week taking care of her, Tomas sends her home. He wonders if he did right to send her away, because in those moments by the sickbed of a woman he hardly knew, he imagined himself in love. He does nothing, but is glad when Tereza calls and tells him she has come to Prague on business. After they meet and make love, Tomas realizes Tereza lied about having business in Prague; far from coming to the city for a brief visit, she had arrived with her entire life packed into one heavy suitcase.
Tereza requires an intimacy Tomas eschewed in the past. When his first marriage ended after two years, Tomas struggled briefly for custody of his son, and then gave up, choosing to break off contact with both his ex-wife and his son. Tomas then began livi ng a light-hearted bachelor existence. An extreme womanizer, he organized his life so that "no woman could move in with a suitcase." He never let his various lovers spend the night.
Tereza breaks Tomas's rules. She arrives carrying a suitcase, literalizing Tomas's fears of dependent women; she spends nights with him, holding his hand as she sleeps. Tomas concludes that making love and sharing sleep with a woman are two different pass ions, and the latter characterizes his love for Tereza. He feels a certain tenderness for Tereza, and compares her to a child sent drifting downstream in a basket for him to find. At this the narrator interjects, and warns that metaphors are dangerous, and Tomas is inviting love.
Despite his tenderness for Tereza, Tomas does not end his other "erotic friendships." Neither does he keep the women in his life apart from each other; for example, he asks his closest friend and mistress, Sabina, to find Tereza a job in a darkroom.
Tereza dreams she must watch Sabina and Tomas make love; when she retells the dream to Tomas, certain details prove she read Sabina's letters. As Tereza learns about Tomas's infidelity, he first denies everything, and then tries to justify himself and ex plain that love and sex are separate for him. Jealous and desperate, Tereza tries overdosing on pills, but Tomas stops her. This passionate attachment is not wholly one-sided; while watching Tereza dance with a male friend, Tomas experiences a modicum of the jealousy that torments Tereza.
Tereza's nightmares continue. Their content overtly expresses Tereza's fear and jealousy of the other women Tomas sleeps with. She dreams cats attack her (it is worthwhile to note that in Czech, the word "cat" is slang for a pretty woman), that Thomas for ces her to perform humiliating acts along with other women, and that she is dead, stripped of her clothes, and plagued by other naked corpses.
Tomas realizes that Tereza is a burden, and that she denies him the privacy he covets, but according to the narrator, his compassion dooms him to take stay with her. Nor does he simply pity Tereza; Tomas finds he can no longer enjoy other women so lighthe artedly. While he does not give up his other sexual partners completely, he finds that if he does not drink before sex, Tereza's image haunts him.
Eventually, Tomas marries Tereza in an effort to make her happy.
Tomas buys Tereza a dog that will become important later in the novel. Although the dog is female, they give her the male name Karenin, after a character in the novel Anna Karenina. The dog grows fiercely devoted to Tereza.
Along with his troubled personal life, Tomas must contend with political turmoil. It is August 1968, and Soviet tanks invade Prague, establishing direct control of the Communist regime. This development puts Tomas in personal danger, because the Communist s object to a paper he wrote years ago. A Swiss doctor offers Tomas a job in Zurich to get him out of the country. For Tereza's sake, Tomas rejects the doctor's offer.
Tereza is happiest during the invasion, roaming the streets with her camera and doing courageous work as a photojournalist. She hands her film to foreign journalists to be published abroad and increase Western awareness of the brutalities taking place. Te reza decides to emigrate. She and Tomas leave their homeland for Zurich.
Sabina also emigrated to Switzerland and has been enjoying artistic success in Geneva. She and Tomas meet in a hotel room; Sabina wears nothing but lingerie and a bowler hat, and the two fall into each other's arms. Tomas continues his infidelities for mo nths after arriving in Zurich. One day, he finds a letter from Tereza, saying that because she knows she is a burden to him, she has left him to return to Prague. Since the Czech borders have been closed in their absence, Tereza's return to Prague means s he will not be allowed to leave the country again.
Tomas attempts to enjoy his newly recovered freedom and tells himself things had to end this way. He lasts five days before he tells his supervising doctor that he must return to Prague. A Beethoven motif plays in his mind: Ess muss sein, or "It mu st be." The narrator notes that Beethoven clearly viewed weight as positive.
In Prague, however, Tomas begins to wonder if he should have stayed in Zurich. He wonders how long his intense longing for Tereza would have lasted; perhaps he could have waited for the pain to subside. Lying next to her in bed, he thinks unhappily that n othing more glorified than chance and accident brought the couple together.
This first section introduces and establishes Tomas's character, and his understanding of his own relationship with Tereza. Tomas seems preoccupied with the same questions that interest the narrator; he ponders, as the narrator does at the beginning of th e novel, the philosophical question of lightness versus weight, and wonders how the question applies to his everyday life. Although Tomas may seem to be a selfish man and an objectionable character, Kundera manages to make him immensely sympathetic. His womanizing is explained less as a conscious choice having to do with conquest, social status or machismo, than as an integral part of his personality and thought. He enjoys lightness and freedom, and resists attempts made by others to trap him. Tomas is a natural individualist. He is also acutely compassionate, however, and cannot passively watch genuine suffering.
Tomas's relationship with Tereza is outlined as a conflict between lightness and heaviness: Tomas wishes to be light, to take his sexuality and exploits lightly and to be free to enjoy his career and interests. Tereza wants a weightier brand of love. She asks for Tomas to devote himself physically and emotionally to her alone. Her intense need for Tomas's devotion prompts her to read his letters and resent the time he spends away from her.
Just as Tereza cannot take love or sexuality lightly, she does not take her work lightly; her dissident work during the occupation interests her passionately, and she seems more attached to her homeland than does Tomas.
Kundera symbolizes Tereza's heaviness by associating her with heavy objects. The suitcase she carries seems to contain her whole life, a burden Tomas must also bear. Tereza also introduces Tomas to Beethoven and the profound heaviness of his music. In add ition, the novel Tereza reads when she first arrives and after which the couple name their dog is Anna Karenina. This novel, with its tragic tale of love, suffering and suicide both foreshadows the kind of love Tereza offers Tomas, and hints constantly at the threat of suicide.
That Kundera portrays both the light Tomas and the heavy Tereza as sympathetic characters points to the sincerity of the philosophical dilemma: both lightness and heaviness seem meaningful and desireable.
Tereza's heaviness begins to seep into Tomas's lightness; he finds a new love for Beethoven, and performs a grandiose, romantic act of sacrifice by returning to his occupied country to be with Tereza. Clearly, Tereza's heaviness fills Tomas's life with a weight and meaning he lacked before. She makes him feel, to some extent, that their life together has meaning. Still, Tomas's long-held belief tells him that life is light; because we cannot make two sets of life decisions and compare the results, the pat h we happened to choose cannot be considered "weighty" or meaningful. He is well aware of the idea that one can never know if one made the right choice, since there is never a point of comparison.
Tomas decides to stay with Tereza because the pair love one another; despite this love, however, they make one another unhappy. This knowledge tortures Tomas, who wonders whether he should have sent Tereza away when she first met him in Prague, or if he should have remained in Switzerland without her.