The story and Chantal and Jean-Marc didn't touch me too much: no, it stayed just a story to me, even though beautifully written at times, it's definitely not Kundera's best work.
Chantal, being older, is said to be the weaker one in the relationship - it is this that triggers a series of anonymous letters sent to her by a secret admirer. However, the two keep misunderstanding each other and it is love that keeps them together when things go really wrong.