Shortly after welcoming her mentor on board, Dr. Crusher notices that he's disappeared and no one else remembers him being on the ship. Soon, Dr. Crusher witnesses more people disappearing and no one can remember them, and a strange vortex is trying to suck her to who-knows-where, possibly to where all the other forgotten people have gone.
The first time I saw this, I was enthralled the whole time. Fortunately, that was before individual episodes were listed on IMDb and before I got a Netflix subscription. In both instances, the mystery of the first two-thirds of the episode is ruined by the description. Though, for those of us who got to see it without knowing what's actually happening, it has a very special place.