Investigating mysterious power failures and a death at an underground research centre, The Doctor discovers a colony of Silurians - prehistoric, intelligent reptiles who went into hibernation before man evolved. But now they have woken up, and they are prepared to wipe out mankind with a killer plague to get their planet back.
| Actor | Role |
|---|---|
 Jon PertweeThe Doctor | The Doctor |
 Caroline JohnLiz Shaw | Liz Shaw |
 Nicholas CourtneyBrigadier Lethbridge-Stewart | Brigadier Lethbridge-Stewart |
 Fulton MackayDr. Quinn | Dr. Quinn |
 Norman JonesMajor Baker | Major Baker |
 Peter MilesDr. Lawrence | Dr. Lawrence |
| TThomasine HeinerMiss Dawson | Miss Dawson |
| JJohn NewmanSpencer | Spencer |
 Geoffrey PalmerMasters | Masters |
| IIan CunninghamDr. Meredith | Dr. Meredith |
 Paul DarrowCaptain Hawkins | Captain Hawkins |
| RRoy BraniganRoberts | Roberts |
| BBill MatthewsDavis | Davis |
| NNancie JacksonDoris Squire | Doris Squire |
| GGordon RichardsonSquire | Squire |
 Pat GormanSilurian | Silurian |
| PPaul BartonSilurian | Silurian |
| SSimon CainSilurian | Silurian |
| JJohn ChurchillSilurian | Silurian |
| DDave CarterSilurian | Silurian |
 Peter HallidaySilurian (voices) | Silurian (voices) |