While stargazing, Prof. Matic discovers a new planet, but it doesn't read on any electronic instruments. Curious, Steve and the others set out in the XL5 to investigate, and find that the planet is more sinister than first thought...
Gerry Anderson
Robert the Robot (voice) (uncredited)
David Graham
Lieutenant Ninety (voice)
Sylvia Anderson
Doctor Venus (voice)
Paul Maxwell
Colonel Steve Zodiac (voice)
John Bluthal
Commander Wilbur Zero (voice)