The two protagonists, Petr and Hana, fail to communicate honestly with each other at critical moments during their experiment with an open relationship. Hana doesn't let Petr know she broke the rules by seeing someone else more than once (because he was really good at sex), and Petr never disclosed his true feelings about his girlfriend having sex with other people until it was far too late.
But the most egregious liar was Petr, by far, because he felt that way from the beginning and hid it from Hana. If only he'd been honest with her every step of the way, they never would've gotten so far down that rabbit hole. He was also quite selfish, indulging in the open relationship when he saw fit without a guilt in the world, but judging Hana for doing the same. She didn't tell him the truth one time, but she was otherwise amazingly open and honest about her escapades whenever they talked afterwards.
The movie has a small cast, but they were all very good in their roles and the critical roles of the two protagonists, who dominate the film, were very well played. It's a slow-moving film and some things didn't work for me, like the endless close-up cell-phone filming between Petr and Hana (a little bit of that would've sufficed), and the too-heavy, too-slow focus on Petr towards the end of the movie. But overall, an effective movie...I can't quite say enjoyable or entertaining because it's a downer of a film despite a fair amount of eroticism.
