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
as The Doctor (1)
Anneke Wills
as Polly
Michael Craze
as Ben Jackson
Patrick Troughton
as The Doctor (2) (uncredited)
Robert Beatty
as General Cutler
David Dodimead
as Barclay
Dudley Jones
as Dyson
Alan White
as Schultz
Earl Cameron
as Williams
Callen Angelo
as Terry Cutler
John Brandon
as American Sergeant
Shane Shelton
as Tito
