Parts: 6
Writer: Bill Strutton
Airdates: 13 February 1965 - 20 March 1965
Production Code: N

Episode Titles: The Web Planet; The Zarbi; Escape to Danger; Crater of Needles; Invasion; The Centre

Doctor: first
Companions: Ian, Barbara, Vicki

Creature Feature: Menoptra and Zarbi

Story: The TARDIS is pulled down to the surface of Vortis, a craggy planet littered with acidic pools and echoing surfaces. Vicki develops a headache from a painful humming sound that only she can hear, so Barbara takes her to bed while the Doctor and Ian explore the planet's surface. Left on her own, Barbara is terrified when her arm seems to take on a life of its own and leads her out of the safety of the TARDIS.

Barbara is diverted from her course by a butterfly-like creature called a Menoptra, who removes a gold bracelet from her arm and frees her from the mind control. Barbara learns that the Menoptra are scared of the Zarbi, ant-like creatures who live on Vortis. The Zarbi, meanwhile, have stolen the TARDIS and taken it to their web-covered base.

When the Doctor is likewise captured and taken to the base, he learns from a mysterious voice that the Menoptra are attempting to invade Vortis, so he agrees to help. But things are not exactly as they appear. The Doctor soon realizes that it is the Animus, a giant parasite, that has invaded Vortis, and the Menoptra are attempting to reclaim their planet. But with few resources at their disposal, and hostile Zarbi swarming all over the surface, the Animus may drain them of their knowledge before they can lift a finger in their defense.