An energy drain traps the Third Doctor and Sarah Jane Smith on the planet Exxilon with its hostile natives, causing the travellers to make an uneasy alliance with a Marine Space Corps expedition and a squadron of Daleks. The key to escape for all of them lies at the heart of a powerful and mysterious lost city, but only after a series of deadly traps.
| Actor | Role |
|---|---|
 Jon PertweeThe Doctor | The Doctor |
 Elisabeth SladenSarah Jane Smith | Sarah Jane Smith |
 Duncan LamontDan Galloway | Dan Galloway |
 John AbineriRichard Railton | Richard Railton |
| JJoy HarrisonJill Tarrant | Jill Tarrant |
| JJulian FoxPeter Hamilton | Peter Hamilton |
| NNeil SeilerCommander Stewart | Commander Stewart |
| AArnold YarrowBellal | Bellal |
| RRoy HeymannGotal | Gotal |
| MMostyn EvansHigh Priest | High Priest |
 Michael WisherDalek Voices | Dalek Voices |
| JJohn Scott MartinDalek | Dalek |
| MMurphy GrumbarDalek | Dalek |
 Cy TownDalek | Dalek |