... race, the memory of them fades. I'm thinking a girl just appeared on the bridge of the Enterprise D, being chased by others of her race (trying to bring her back) when she asks for help and basically tells the crew that they had just spent lots of time with her people. Her and Riker had fallen in love and that's why she let her people to seek out the Enterprise (a big no-no) and the crew had already forgotten her and her people. She returns with her people at the end and Riker hand writes a journal about her as computer files also fade and he didn't want to forget her.
Too much for me to have imagined it, but I can't find the episode or anything like it. I swear it wasn't a book... Any ideas?