Star Trek: Season 3, Episode 8 - ProjectFreeTV

3 - 8: For the World is Hollow and I Have Touched the Sky

After Dr. McCoy informs Captain Kirk that he is dying, the Enterprise crew discovers a planetoid that is on a collision course with a inhabited world. Kirk, Spock and McCoy beam down and discover it is a populated space ship where most of the inhabitants having no clue they are on a space ship.