3rd Rock from the Sun - Season 6 Episode 11 : A Dick Replacement 2001

Dick and Mary visit a psychic together, who claims that the pair are from two different worlds and predicts that Dick will one day up and leave Mary. Horrified at this news, Dick decides to find a replacement boyfriend for her, settling on a man called Gary; who thinks he's actually being offered Dick's position of physics professor. Meanwhile, Sally and Harry decide that if the psychic really knows that they are aliens, then they must kill her, but in a way that isn't possible for her to predict.