The Doctor's TARDIS lands at the Snowcap space tracking station in Antarctica in December 1986. A routine space mission starts going wrong. When the base personnel's suspicions are roused, the Doctor informs them that the space capsule is being affected by the gravitational pull of another planet — a tenth planet in the Solar system.
William Hartnell
The Doctor (1)
Anneke Wills
Polly
Michael Craze
Ben Jackson
Patrick Troughton
The Doctor (2) (uncredited)
Robert Beatty
General Cutler
David Dodimead
Barclay
Dudley Jones
Dyson
Alan White
Schultz
