John Robinson is trapped in a cave inhabited by an evil spirit that possesses him. Once rescued by his family, he begins to show a bipolar personality where he can appear friendly at one moment and the next minute behave in an aggressive and despotic manner. Every time I watch the episodes of the first season, and this one in particular, I regret that the series has not continued down this path. For me this is the best episode of the entire series, Guy Williams is outstanding here, the rest of the cast too. Here we have the perfect combination, a disturbing story, the entire cast doing an excellent job, an almost terrifying atmosphere, a correct direction. It is an episode that I never get tired of watching. It's a shame that the influence of Batman has ended up turning a science fiction and adventure series into a fantastic comedy in the following seasons and that characters as rich as Dr. Smith end up being something of a clown.