On the planet Dido, the Doctor, Ian and Barbara meet two survivors from a crashed spaceship who are terrorised by the monster Koquillion.
| Actor | Role |
|---|---|
 William HartnellThe Doctor | The Doctor |
 Jacqueline HillBarbara Wright | Barbara Wright |
 William RussellIan Chesterton | Ian Chesterton |
 Maureen O'BrienVicki | Vicki |
 Ray BarrettBennett / Koquillion | Bennett / Koquillion |
| TTom SheridanSpace Captain (voice) | Space Captain (voice) |
| JJohn StuartDidonian (uncredited) | Didonian (uncredited) |
| CColin HughesDidonian (uncredited) | Didonian (uncredited) |