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.