Ep 18. Escape to Danger (3)

The TARDIS arrives on the planet Vortis. There the Doctor and his companions discover and become involved in the plans of butterfly-like creatures (Menoptra) to reclaim their planet from a spidery parasite that is steadily enveloping it in a web-like structure. They join up with a party of Menoptra and their allies (the underground-dwelling Optera) and manage to destroy the parasite.