Doctor Who: Superior Beings

The planet Rocosia consists of nothing but immaculately-cultivated plant life, which propagates and cultivates itself -- there are no mammals, insects, aquatic or avian creatures living there. So the plants seem to exist for no apparent ecological reason, but visitors to the planet are soon distracted from the enigma by the dazzling beauty of the gardens.One night, Peri spots a strange, multi-limbed creature picking fruit from one of the trees, and she decides to follow it. She gets lost though, and ends up attacked by a Decimator, a vicious, thorny plant. Regaining consciousness, she finds herself the captive of a humanoid fox in tight-fitting leather who is clearly very excited about eating her...Meanwhile the Doctor finds Rocosia in a state of chaos as the Decimators appear in force and embark upon systematic destruction of the entire planet. The Doctor is rescued by the Valethske -- the creatures holding Peri captive. What are the Valethske doing on Rocosia and why are the Decimators trying to destroy the planet? And will the Doctor and Peri be able to escape before the entire planet is consumed by fire?