On the planet Vortis, the Doctor, Ian, Barbara and Vicki are swept up in the struggles of the butterfly-like Menoptera, the original denizens of Vortis who were forced to flee the planet for the moon Pictos to escape the encroaching web of the Animus and its mind-controlled minions, the ant-like Zarbi and their living weapons, the larvae guns.

On the planet Vortis, the Doctor, Ian, Barbara and Vicki are swept up in the struggles of the butterfly-like Menoptera, the original denizens of Vortis who were forced to flee the planet for the moon Pictos to escape the encroaching web of the Animus and its mind-controlled minions, the ant-like Zarbi and their living weapons, the larvae guns.