We meet Tomas's lover Sabina again, this time just as she is joined by another of her lovers, Franz, a good-looking and guilt-ridden married professor. He suggests a trip to Palermo, but Sabina refuses. She pours them wine, and removes her clothing; she leads Franz to the mirror, and places the bowler hat on her head. Franz, thinking she is joking, removes the hat. He kisses her, she agrees to go to Palermo, and he leaves the studio.
Alone and disappointed, Sabina remembers the first time the bowler hat entered her lovemaking. In Prague, Tomas put it on her head as a joke, and both looked at her image in the mirror, growing aroused. Sabina thinks the hat undermines her dignity as a woman. She thinks about wearing the hat in her lovemaking with Tomas, and muses it was "far from good clean fun it was humiliation"— a humiliation she provoked and enjoyed.
The bowler originally belonged to Sabina's grandfather, a mayor in a small Czech town, and was then passed down to Sabina's father. After her parents' death, rather than fight her brother for inheritance rights, Sabina told him she would just take the hat. The bowler hat became an erotic object for her and Tomas, and eventually came to symbolize their affair, and their time in Prague. Now it begins to stand for Franz's misunderstanding of Sabina.
The narrator comments that Franz and Sabina, like any pair of lovers who meet later on in life, have a dictionary of mutually misunderstood words. He devotes the middle section of Part 3 to defining these misunderstood words. A sampling:
Woman : Franz bases his ideal of womanhood on his mother. Franz considers Sabina a woman in this sense; Sabina considers herself "Sabina", an non-gendered essential being. She thinks of her femininity as secondary, an accident of birth not crucial to her identity.
Fidelity and betrayal: Franz worships fidelity. He loved his mother faithfully until she died. He hates betraying his wife to be with Sabina. Sabina, on the other hand, finds betrayal interesting. Her life is a series of betrayals—of family, art school, country and lovers.
Music: Franz loves music and finds it intoxicating; he makes no distinction between classical and rock music. He longs for music to obliterate the need to speak and search for precise words, as he has been doing all his life. Sabina hates music she thinks of as mere noise (most music comes under this heading for Sabina); she associates noisy music with loud Communist youth summer camps.
Lightness and darkness: Franz appreciates darkness; he closes his eyes during intercourse, because this makes him feel that he is approaching infinity. To Sabina this makes him seem lifeless, and rather than watch him, she closes her eyes too. As a visual artist, Sabina associates seeing with living. She does not like either complete darkness or blinding light and in general stays away from extremes. Parades: Franz loves parades. In Paris, where he studied, he took part in every possible demonstration, feeling he was part of the Grand March of the European left. Because he spent most of his time inside, studying or lecturing, parades make Franz feel connected to real people and a more real life. Sabina, a supreme individualist forced to take part in Communist parades early on, can think of nothing worse than parades.
The beauty of New York: Franz and Sabina agree that New York's beauty consists of its accidental, unpremeditated oppositions and mistakes. Although he finds New York beautiful, New York makes Franz uneasy and instantly homesick. Sabina likes the accidental quality of New York's beauty; it reminds her of her paintings.
Sabina's country: Franz admires Czechoslovakia and the spirit dissidents show during repression and revolutions. He finds Czechoslovakia deeply romantic and sees its people living the dramatic "real life" he has never experienced. Sabina has no taste for this drama and thinks it ugly and devoid of romance.
Cemetery: To Franz, a cemetery is an "ugly dump of stones and bones." The only Czech places Sabina longs for are the cemeteries, which she thinks of as peaceful, flowering gardens where silence can be found even in wartime.
The old church in Amsterdam: Franz is fascinated by the building because "the Grand March of History had passed through the hall." He sees the emptiness and thinks of the relief it must have been to sweep out all of the collected artifacts. He thinks he may need to simplify his life as the building was simplified. Sabina recalls Communists doing the same thing to Czech churches and thinks that the link is a hatred of beauty. She remembers escaping a work brigade to sit in a village church, and fin ding the church very beautiful compared to the world outside.
Strength: Franz laughs at his own physical strength and considers it useless in his city life. He believes love "means renouncing strength." Sabina admires his strength and wishes it translated internally, or that he would use his strength on her.
Living in truth: Franz views privacy, including secrecy in love, as a form of deception. In order to feel that he lives in truth, he decides to leave his wife and live with Sabina. For Sabina, on the other hand, living in truth means living privately, not allowing the public to see or interfere in one's life. A secret love is the only kind she could conceive of.
Franz's wife Marie-Claude holds a dinner party for local artists. Her loud, vulgar hostessing offends Franz. Sabina enters and Marie-Claude accosts her, commenting loudly on the ugliness of Sabina's pendant. (She says this to put the beautiful Sabina in her place, not because she suspects that Franz and Sabina are having an affair).
Franz thinks of his admiration for the simple Amsterdam church, and decides to sweep his life clean. He packs his bags and tells Marie-Claude he has been having an affair. She reacts neither with pain nor surprise, but with quiet aggression. Franz leaves to meet Sabina.
Sabina tries to convince herself she must put an end to her lifelong pattern of betrayal, but she feels increasing distaste for Franz. That night when they make love and Franz again closes his eyes, she thinks of him as a grotesque oversized newborn. Both are drunk with joy, Franz because he thinks he will spend the rest of his life with Sabina, Sabina because she has resolved to leave Franz the next day.
Franz finds out Sabina has left and understands nothing. He finds a small apartment and starts living alone for the first time in his life. Preferring the ideal to the real, he finds happiness imagining Sabina as a guiding presence in his life. He starts living with one of his students, a kind young girl with thick glasses. Franz's wife refuses to grant him a divorce, but he lives happily with his new mistress, sharing his ideals and dragging her to Czech dissident émigré conferences.
Sabina moves to Paris. She wonders where to go next, and if her betrayals will lead to death. Tomas's son informs her by letter that Tomas and Tereza have died. Sabina thinks of death. To her no more terrifying thought exists than the image of a heavy stone trapping her underground.
Franz and Sabina represent such extremes of heaviness and lightness that they fail to understand each other. Unlike Tereza and Tomas, who torture each other but more or less understand how the other feels, Sabina and Franz interact like members of a different species.
Sabina is so light that she betrays again and again. While she is tired of running away and would like to meet someone strong enough to hold her down, and while she understands that Franz is the best man she has ever known, Sabina reacts to heaviness with instinctive distaste. And Franz, the man she almost loves, is supremely heavy. As Tereza does, Franz imbues everything in his life with profound meaning. Even Tereza doubts that meaning sometimes, but Franz does not entertain doubts.
The unbearable lightness of being excites Sabina, and she is excited by the thought of betraying such a good loving man for no meaningful reason.
Even though characters and human beings can follow only one path, authors have more flexibility; Kundera, for example, can experiment and see what happens when different choices are made. This seems to be Kundera's (serious) joke: he takes two couples, both composed of one light and one heavy person, and makes them do different things. Thus he gives the reader what the characters complain they lack: the ability to compare paths. Tomas and Tereza decide to stay together; Sabina and Franz part ways.
Kundera also uses Franz to level a devastating and cynical critique against the European left. Leftist beliefs are painted as the ultimate naive idealism on the part of scholars who have spent their lives indoors and understand nothing of revolution's horrors.