Kundera is one of the most enjoyable Central/East European authors of the post-war period and that is because he wrote a number of books with a very simple, and flavorful, message: we fight totalitarianism by simply not caring about it; adultery is one very good way to be a dissident. Ludvik Jahn served in a non-combatant military unit, then spent a year in a prison (without conviction) and had to work for six years in the mines (for lack of university degree) just because his colleagues and Party comrades took very seriously a stupid joke he wrote to his beloved for absolutely personal reasons. Jahn's revenge had to take the form of a joke because his major concern was namely that: how could these men take so seriously an innocent charade? Yet, his joke turned out bad: instead of humiliating Pavel, the once leader of the students' party organization, by seducing his wife, Jahn humiliated the innocent, and naive, woman, broke the heart of her young suitor and, most unfortunately, had to realize that Pavel, the allegedly serious communist, fared much better in the field of adultery: enjoying the company of 20-year old attractive students who have absolutely no notion of Marxism and the construction of socialism. It is through this realization, and not so much through his earlier ostracization, that Ludvik is confronted with the consequences of his own misplaced joke. His lonely protest against the system failed.
ADDED IN 2009: now, that Kundera was revealed to have himself denounced a "capitalist spy" to the police authorities and thus contributed to the long-term prison/correction camp sentences of several people, this book/movie develops in an unexpected dimension. Was it a deliberate, or subconscious way for Kundera to deal with his own guilt, a way to explain to his younger self that what he did, apparently out of good faith and sense of civic duty, at the age of 20, seemed nothing but utter stupidity at the age of forty?