Norman, an android pretending to be a member of the U.S.S. Enterprise crew, takes control of the starship and her crew, taking them to an unnamed planet where Kirk discovers an old nemesis, Harry Mudd and a planet populated entirely by androids.

Roger C. Carmel (Harcourt Fenton "Harry" Mudd)
Richard Tatro (Norman)
Alyce Andrece (Alice #1 through 250)
Rhae Andrece (Alice #251 through 500)
James Doohan (Lt. Cmdr. Montgomery Scott)
Nichelle Nichols (Lt. Uhura)
George Takei (Lt. Hikaru Sulu)
Walter Koenig (Ensign Pavel Chekov)
Kay Elliot (Stella Mudd)
Mike Howden (Lt. Rowe)
Michael Zaslow (Ensign Jordan)

• The single Norman model controls the entire population of androids.
  
• Kirk causes Norman to overload by telling him that Harry Mudd is a liar. When Harry tells Norman "I am lying", it throws Norman into a "does not compute" loop complete with smoke pouring out of his head.
  
• The newly upgraded Main Engineering set is first seen in this episode, after a preview in "The Doomsday Machine".
  
• Harry Mudd previously appeared in the episode "Mudd's Women".