ST:DS9, The Passenger, Episode 109
There sure are a lot of Star Trek episodes that focus around the concept of possession.
Bashir is taken over by a mass murder who has figured out a way to pass his consciousness from one body to the next.
Oh. Right. Spoilers. Oops.
Episode was sort of stilted and awkward. But I guess it gave Alexander Siddig the chance to fire a phaser and kill someone, where (as the chief medical officer) he may never do so.
