Affected by a strange force, the TARDIS is dragged down to the desolate planet of Vortis. Until they can discover what is holding them there, the Doctor and his friends are trapped on the planet. The Doctor, Ian and Vicki are captured by the Zarbi – huge ant-like creatures controlled by the parasitic alien Animus. Meanwhile, Barbara runs into a group of Menoptra, butterfly-like creatures that have been driven from their home planet by the Animus, and plan to return with an invasion force. But the Zarbi know their plans and are waiting for the Menoptra... This novel is based on a Doctor Who story which was originally broadcast from 13 February-20 March 1965.