The Joke is Kundera's first novel and features four primary characters: Ludvik Jahn (the primary protagonist), Jaroslav, Helena, and Kostka. The novel does not contain a standard narrative and is best understood with respect to the plot lines concerning these four characters.

The majority of the book is rooted in Ludvik's two hapless "jokes." The first is in a postcard to his friend Marketa. Marketa is at a youth camp for young communists and, somewhat annoyed by the earnest tone in her letters, Ludvik, then a young student, sends her a postcard that says "Optimism is the opium of the people! A healthy atmosphere stinks of stupidity! Long live Trotsky!" Even though Ludvik is a member of the Communist Party, the Stalinist powers that be do not take kindly to his joke. His own friend reports him and, after being expelled from the university, he is compelled to penal servitude in mines. After all this—which takes place in the 1950s—Ludvik attempts to get his revenge by playing another "joke," this time on his erstwhile friend, Pavel. He does so by planning an affair with Pavel's wife, Helena. Unfortunately this joke, too, backfires since Pavel is only to happy to have someone who will take Helena off his hands.

The other characters also have their own "jokes" that shape the plot.

Kostka, who serves as a foil to Ludvik, separates himself from the Communist Party because of his Christian faith. Kostka thinks that it is possible to reconcile Christian faith with Communism; however, in thinking this he strays very far indeed from the party line.

Jaroslav cares deeply about Moravian folk culture and is very troubled by the Party's appropriation of it. His own wedding is described as a "showcase of traditional rituals and customs." Jaroslav wishes to revive a folk tradition called the Ride of the Kings; when he was young, he played the king. He learns, later, that his son has been chosen to be the king in the ritual, and this pleases him immensely. The joke is revealed when we learn that the king in the ceremony is not, in fact, his son. Neither his son nor his wife share his passion for folk culture, and they conspired so that Jaroslav's son could be away at a motorcycle race instead of at the outmoded ritual that his father cares about so passionately.