What is the main conflict in The Joke by Milan Kundera?

When his girlfriend goes away to a special training camp sponsored by the Party, she writes him that she is having a wonderful time, embracing all of the Party doctrine. He writes her a postcard and includes a sarcastic remark seemingly against the Party on the card. He does not hear from his girlfriend for the rest of her time at the Camp, save a terse, perfunctory review of what she is doing. Meanwhile, Party members have seen the postcard and decide to eject Jahn from the Party and all of its benefits there within. He is kicked out of university and his entire life destiny is washed away in moments.