It describes how a student's private joke derails his life, and the entwined stories of his lovers and friends grappling with the shifting roles of folk traditions and religion under Communist Czechoslovakia.

The novel opens with Ludvik back in his hometown in Moravia for the first time in years, startled to recognize the woman cutting his hair, though neither acknowledges the other.

During their summer break, a girl in his class wrote to him about "optimistic young people filled through and through with the healthy spirit" of Marxism; he replied caustically, "Optimism is the opium of mankind!

Commissions were convened to investigate Ludvik, who remained defiant, culminating in a plenary session — led by his peer, Pavel Zemanek — in which he was unanimously expelled from the Party and from the college.

He sees an opportunity for revenge when he meets the radio reporter Helena and learns she is married to Zemanek, who had led Ludvik's expulsion from the Party.

His arranges to borrow the apartment of Kostka, who had himself been pushed out of the Party for his Christian faith; years prior, Ludvik had helped him find a good job.

Ludvik meets with Kostka, who confirms that the girl in the barbershop had been Lucie, and goes on to say that he knew her well: he had learned of her past traumas, converted her to Christianity, and cheated on his wife with her.

His revenge on Zemanek thwarted, his memory of Lucie confused, Ludvik wishes he could erase his years of pointless mistakes, and wants to escape Helena and his hometown as soon as possible, but he misses his train.

Helena believes she will be leaving with Ludvik, but he, incapable of explaining the tortuous reasoning of his cruel stunt, simply tells her he does not love her and will never see her again.

Ludvik, assuming Jaroslav will survive, imagines the second half of his friend's life will be dimmer, and realizes that "one's destiny is often complete long before death."

Kundera was dismayed to realize it was heavily modified and abridged, with a different number of parts; unable to leave the country, he complained in a letter to the Times Literary Supplement.

"[2] Philip Roth called it "[a] direct and realistic book, openly reflective about the issues it raises—proceeding by means of philosophical thoughtfulness and accurate observation of a fairly broad spectrum of 'politicized' citizens, it is something like a cross between Dos Passos and Camus".

Released in February 1969, some six months after the August 1968 Warsaw Pact invasion that ended the Prague Spring, the film was initially successful in theaters before it was pulled from distribution and banned for the next 20 years.