While it definitely isn't original, there is no one ep I can point to and say, "This is where it came from." I mean, some of it sort of smacks of "Masks" or something, or maybe "Turnabout Intruder", but it's not any of those things.
Mind you, it doesn't exactly work, and I cannot for the life of me figure out how Trip got posessed at the end. But for Enterprise, just making it past that first hurdle - a somewhat original story - is a big step.
Especially with my annual viewing of "Oasis" coming up.