Set in Prague in 1968, the novel details the circumstances of the lives of artists and intellectuals in Communist Czechoslovakia in the wake of the Prague Spring, and the subsequent invasion by the USSR. The major protagonists include: Tomáš, a well-known, successful surgeon; his wife Tereza, a photographer in anguish over her husband's many infidelities; Tomáš' lover Sabina; Sabina's lover, Franz; Simon, Tomáš' estranged son from a first marriage.
The book centers on Nietzsche's idea of eternal return - that is, the idea that the universe and all the events therein have all happened before, and will continue to recur ad infinitum. Kundera challenges this idea, offering an alternative: each of us has only one life to live, and what happens once will never occur again. He calls this idea "lightness", and refers to the concept of eternal return as "heaviness" or "weight".