What appears to be a Yellow Fever epidemic breaks out at an exclusive health clinic on one of the Scilly Isles. After the island is sealed off from visitors and any possible mosquitoes exterminated, Doomwatch is granted permission to investigate, but Quist's team soon discovers that the illness is actually a mutated new virus. It seems that an experiment into vaccines has gone wildly out of control, and hundreds of spiders are now carrying a deadly new disease.