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.
| Actor | Role |
|---|---|
 William HartnellThe Doctor (1) | The Doctor (1) |
 Anneke WillsPolly | Polly |
 Michael CrazeBen Jackson | Ben Jackson |
 Patrick TroughtonThe Doctor (2) (uncredited) | The Doctor (2) (uncredited) |
 Robert BeattyGeneral Cutler | General Cutler |
| DDavid DodimeadBarclay | Barclay |
| DDudley JonesDyson | Dyson |
| AAlan WhiteSchultz | Schultz |
 Earl CameronWilliams | Williams |
| CCallen AngeloTerry Cutler | Terry Cutler |
 John BrandonAmerican Sergeant | American Sergeant |
| SShane SheltonTito | Tito |
 Steve PlytasWigner | Wigner |
| CChristopher MatthewsRadar Technician | Radar Technician |
| EEllen CullenGeneva Technician | Geneva Technician |
| CChristopher DunhamR/T Technician | R/T Technician |
 Glenn BeckTV Announcer | TV Announcer |
 Roy SkeltonCybermen (voice) | Cybermen (voice) |
 Peter HawkinsCybermen (voice) | Cybermen (voice) |
| HHarry BrooksCyberman | Cyberman |
 Reg WhiteheadCyberman | Cyberman |
| GGregg PalmerCyberman | Cyberman |