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.
Elenco: William Hartnell , Patrick Troughton , Jon Pertwee , Tom Baker , Peter Davison , Colin Baker , Sylvester McCoy .