The TARDIS is seized by a mysterious force and dragged down to its latest resting place - Vortis, a craggy, forbidding world where the dangers include pools of lethal acid. Here, the Doctor, Ian, Barbara and Vicki get involved in the struggles of the moth-like Menoptra (Roslyn de Winter, Arne Gordon, Arthur Blake, Jolyon Booth, Jocelyn Birdsall, Martin Jarvis) to reclaim the planet - originally their home - from an alien parasite. The Animus (voiced by Catherine Fleming) has invaded Vortis and taken control of the once-docile, ant-like Zarbi (Robert Jewell, Gerald Taylor, John Scott Martin, Kevin Manser), turning them into its vicious drones who use organic Venom Guns (Jack Pitt, Hugh Lund) to enforce the Animus' will.

Eventually, with the time travellers' help, a group of Menoptra and Optera (Ian Thompson, Len Russell, Jane Bowman, Barbara Joss) - stunted troglodyte descendants of the planet's original Menoptra inhabitants - manage to infiltrate the Animus's web-city, the Carsenome, and confront the creature directly, destroying it with a weapon, the Isop-tope, devised by their scientists.

_{Synopsis from Doctor Who: The First Doctor Handbook by David J. Howe, Mark Stammers and Stephen James Walker, reprinted with permission; further reproduction is not permitted. Available from Telos}