Greatest Moments in Geekdom:
Buck Rogers: Space Vampire, Season 01, Episode 14
Buck Rogers went from sci-fi to horror in this episode where Wilma was being stalked by a space vampire. I remember being completely horrified by this episode as a child (even though the vampire had a ridiculously massive unibrow), specially the part where Wilma gets possessed and the Vorvon screams as he meets his final fate.
Watch the full episode for FREE on NBC.com.