Dr Smith, Will and the Robot discover an old harp under a carved out rock. They discover that in playing the right tune, it unlocks a passage to another world. In travelling here, Dr Smith meets an alien who believes to be the devil, and believes the place to be Hell. When he leaves, and the Alien shows up on the planet, Dr Smith is frightened. Judy befriends the alien, and is accidently taken to the other world. It is up to Don and John to save her, and finally Dr Smith realises it is not hell, and merely a prison world for a criminal.