A strange power drain forces the TARDIS to materialise on the planet Vortis. There, the Doctor, Ian, Barbara and Vicki become involved in the plans of the butterfly-like Menoptra to reclaim their planet from the parasitic Animus that is slowly enveloping it with its web-like domain, the Carsenome.

Joining forces with an advance party of Menoptra and the underground-dwelling Optra, the travellers manage to destroy the Animus, freeing the ant-like Zarbi and death-spitting larvae guns from its control and releasing the TARDIS from its influence.