On the lush planet Tigella, two opposing factions are divided over the usage of the Dodecahedron, an ancient and powerful artefact which provides the entire planet's energy. The Doctor is summoned to arbitrate the conflict, but the power-crazed Meglos intercepts the call and impersonates him in order to steal the Dodecahedron.
| Actor | Role |
|---|---|
 Tom BakerThe Doctor / Meglos | The Doctor / Meglos |
 Lalla WardRomana II | Romana II |
 John LeesonK9 (voice) | K9 (voice) |
 Bill FraserGeneral Grugger | General Grugger |
 Frederick TrevesLieutenant Brotadac | Lieutenant Brotadac |
 Edward UnderdownZastor | Zastor |
 Jacqueline HillLexa | Lexa |
| CColette GleesonCaris | Caris |
| CCrawford LoganDeedrix / Meglos (voice) (uncredited) | Deedrix / Meglos (voice) (uncredited) |
| CChristopher OwenEarthling | Earthling |
| SSimon ShawTigellan Guard | Tigellan Guard |