OK, I'm probably the only person on the boards who'd know this one:
The entire episode of "Imaginary Friend," from
Star Trek: The Next Generation. (Sorry, I can't remember the season number-- usually I try to put info like that in.) In it, a little girl's imaginary friend is taken over my a bodiless alien thing and begins wreaking (sp?) havoc on the ship.
I doubt I'll ever forget Whoopi Goldberg's line best line in the ep (IMO):
I don't think you should have to give up your imaginary friend. I automatically thought of Foster's and Mac and Bloo. (

&

)