Space: 1999 - Season 2 - Eps 11: Seed of Destruction
Whilst he is exploring an asteroid Koenig is taken over by an alien race known as the Kalthon, who replace him with an exact double. In consequence every action ordered by the fake Koenig is solely for the benefit of the Kalthon and to the detriment of the welfare of the Alphans. Helena realizes that something is wrong when 'Koenig' is cold to the touch and needs to convince the other crew members.
About Space: 1999

Title: Space: 1999
First Air Date: 1975-09-04
Last Air Date: 1977-11-12
Status: Ended
Rating: 7.1/10 (from 140 votes)
Language: EN
Seasons: 3
Total Episodes: 48
Network: ITV1
Genres: Sci-Fi & Fantasy
Production Companies: Group 3, RAI, ITC Entertainment, RAI Cinema
Synopsis
The crew of Moonbase Alpha must struggle to survive when a massive explosion throws the Moon from orbit into deep space.
Cast

Martin Landau
John Koenig

Barbara Bain
Helena Russell

Catherine Schell
Maya

Tony Anholt
Tony Verdeschi