The Fifth Doctor, Tegan and Turlough arrive on Sea Base 4, a nuclear warhead station under the sea that has some very nasty neighbours.
| Actor | Role |
|---|---|
 Peter DavisonThe Doctor | The Doctor |
 Janet FieldingTegan Jovanka | Tegan Jovanka |
 Mark StricksonVislor Turlough | Vislor Turlough |
 Tom AdamsVorshak | Vorshak |
 Ian McCullochNilson | Nilson |
 Ingrid PittSolow | Solow |
| NNigel HumphreysBulic | Bulic |
| TTara WardPreston | Preston |
| MMartin NeilMaddox | Maddox |
 Nitza ShaulKarina | Karina |
 James CoombesParoli | Paroli |
 Norman ComerIcthar | Icthar |
| SStuart BlakeScibus | Scibus |
| VVincent BrimbleTarpok | Tarpok |
| CChristopher FarriesSauvix | Sauvix |
| LLing TaiSeabase Crewmember | Seabase Crewmember |