I seem to remember an episode where a woman appeared on the bridge asking for help as if she knew the crew and was in love with one of them. (Riker? Chakotay?) However this race is forgotten by outsiders when they are a distance away from them.
The Star Trek ship has just spent considerable time getting to know these people but after parting (and our heroes forgetting, including computers records, which also faded) the lady love left her people to run back to the Federation ship (forbidden by her people). Her people were chasing her, demanding her return, but she didn't want to go. She ended up going back and because of the memory thing, her love interest hand wrote a journal entry about her so that he would not forget.
Who are these people, which series was it and what episode?