 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.