Complex sentence: Mondas is first mentioned in the 1966 "Doctor Who" serial "The Tenth Planet"; this episode is also the first appearance of the Cybermen, a race that recurs in numerous "Doctor Who" stories.
Split into simpler sentences: Mondas is first mentioned in the 1966 "Doctor Who" serial "The Tenth Planet". This episode is also the first appearance of the Cybermen, a race that recurs in numerous "Doctor Who" stories.