The Doctor, Romana and K-9 arrive on the forest planet Tygella, where the native Tygellans now live in a underground city, due to the carnivorous plants. Where the Doctor finds he has been wrongly accused of stealing the Dodechedron, the city's power source. Only to discover the culprit is Meglos, the cactus-like last survivor of the desert planet Zolfa-Thura and he bids to return the Dodechedron to Zolfa-Thura for his own evil purpose and plans to use it as a powerful weapon and he plans to wipe out Tigella.

With the dodecahedron gone, the city goes power critical. Its death, imminent. With Romana seeking escape from Meglos' pirates, and Meglos seeking escape from the sealed-up city, and the Doctor himself seeking to understand what's been going on (and why everybody wants him arrested), religious factions move in to take over the city.