On the planet Alzarius in the alternate universe of E-Space, the Doctor, Romana and K9 find survivors of a crashed starliner, strange creatures lurking in the marshes and a genetic riddle which stretches back centuries.
| Actor | Role |
|---|---|
 Tom BakerThe Doctor | The Doctor |
 Lalla WardRomana | Romana |
 Matthew WaterhouseAdric | Adric |
 John LeesonK9 (voice) | K9 (voice) |
 George BakerLogin | Login |
 Leonard MaguireDraith | Draith |
 James BreeNefred | Nefred |
 Alan RoweGarif | Garif |
| TTony CalvinDexeter | Dexeter |
 Richard WillisVarsh | Varsh |
| BBernard PaddenTylos | Tylos |
| JJune PageKeara | Keara |
 Andrew ForbesOmril | Omril |
| AAdrian GibbsRysik | Rysik |
| BBarney LawrenceMarshman | Marshman |
| NNorman BaconMarshchild | Marshchild |