The Zarbi, huge ant-like creatures with metallic bodies and pincer claws, are waiting for Tardis [sic] when its police-box shape materializes on the cold and craggy planet Vortis. They capture Doctor Who, Ian and Vicki and take them to their weird Headquarters, a city of web-like organic matter.

But the Zarbi are not the only beings on Vortis. Barbara has fallen into the hands of the butterfly-like creatures with soft voices and iridescent wings, whose civilisation has been destroyed by the Zarbi. She learns that her captors are only the advance party of Menoptera in exile who plan to win back their planet by an invasion from outer space. For the Zarbi 'have brought the dark age to Vortis'.

In the final thrilling chapters, Doctor Who and the crew of Tardis encounter the power which controls both the Zarbi and the living Web City. How can they defeat this strange bladder of blazing light which draws in and absorbs all who come into its presence?