Basically, Kirk fell in love with a woman and acted totally out of character, including putting his crew into danger, and in the end he was responsible for her death (turned out she was an Android though). Back at the ship, he regrets his actions and stupidity and says "I wish I could forget". After he has fallen asleep, Spock takes away his memories. And then the episode just ends like that.
It upset me so much that I actually am writing a fanfiction at the moment where Kirk finds out what he did and feels he can't trust him anymore. I think it was wrong, Spock should have at least asked if that is what he really wants. We often say things we don't really mean? In one of the later movies Kirk was offered something similar but refused of having his pain taken away, saying "I need my pain!" and I agree, how can we grow and learn if we just forget the bad things we did? I'm probably 50 years too late for this discussion, but I really feel there should have been something more at the end of the episode.