On the planet Vortis, the Doctor and his companions aid the butterfly-like Menoptra in their battle against the parasitic Animus and its mind-controlled minions, the ant-like Zarbi.
| Actor | Role |
|---|---|
 William HartnellThe Doctor | The Doctor |
 Jacqueline HillBarbara Wright | Barbara Wright |
 William RussellIan Chesterton | Ian Chesterton |
 Maureen O'BrienVicki | Vicki |
| CCatherine FlemingAnimus (voice) | Animus (voice) |
| RRoslyn de WinterVrestin | Vrestin |
| AArne GordonHrostar | Hrostar |
| AArthur BlakeHrhoonda | Hrhoonda |
| JJolyon BoothPrapillus | Prapillus |
| JJocelyn BirdsallHlynia | Hlynia |
 Martin JarvisHilio | Hilio |
| IIan ThompsonHetra | Hetra |
| BBarbara JossNemini | Nemini |
| RRobert JewellZarbi | Zarbi |
| JJack PittZarbi | Zarbi |
 Gerald TaylorZarbi | Zarbi |
| HHugh LundZarbi | Zarbi |
| KKevin ManserZarbi | Zarbi |
| JJohn Scott MartinZarbi | Zarbi |