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
• Anneke Wills
• Michael Craze
• Patrick Troughton
• Robert Beatty
• David Dodimead
• Dudley Jones
• Alan White