The Doctor and his companions Steven Taylor and Dodo Chaplet arrive some ten million years into the future, on board a generation starship which is carrying the last of humanity away from an Earth that is about to fall into the Sun. However, the cold that Dodo has could prove devastating to these future humans and their servants, the Monoids.
| Actor | Role |
|---|---|
 William HartnellThe Doctor | The Doctor |
 Peter PurvesSteven Taylor | Steven Taylor |
 Jackie LaneDodo Chaplet | Dodo Chaplet |
| EEric ElliottCommander | Commander |
| IInigo JacksonZentos | Zentos |
| RRoy SpencerManyak | Manyak |
 Michael SheardRhos | Rhos |
| KKate NewmanMellium | Mellium |
| IIan FrostBaccu | Baccu |
| SStephanie HeesomGuardian | Guardian |
 Paul GreenhalghGuardian | Guardian |
| EEdmund CoulterMonoid | Monoid |
| FFrank GeorgeMonoid | Monoid |
| RRalph CarriganMonoid | Monoid |
| JJohn CaesarMonoid | Monoid |
 Roy SkeltonMonoid Voice | Monoid Voice |
| JJohn HalsteadMonoid Voice | Monoid Voice |
| TTerence WoodfeldMaharis | Maharis |
| BBrian WrightDassuk | Dassuk |
 Eileen HelsbyVenussa | Venussa |
 Terence BaylerYendom | Yendom |
| RRichard BealeRefusian Voice | Refusian Voice |