Will becomes increasingly concerned when the Jupitar 2 becomes enveloped by a strange blue mist and members of his family inexplicably begin to disappear. The story, which borrows from 1956's 'Forbidden Planet', doesn't make much sense (notably the much commented on new third deck on the Jupitar 2 or the various mysterious manifestations (such as giant space-hands) that seem unaccountable given the premise), and the 'alien threat' is Dawson Palmer draped in a sheet, but, despite these weaknesses, the episode is pleasantly moody and unexpectedly entertaining. Smith-silliness is kept to a minimum and Johnathon Harris actually pulls-off 'menacing' as his character (or a doppelganger thereof) pursues young William through the ship's new addition. Considering the nonsense Irwin Allen was peddling in the show's third season, 'The Space Creature' is reasonably entertaining (if you don't think about it too much and are not looking forward to seeing an actual 'space creature'). "Id" referred to as a "big word" is one of the series' cleverer jokes.