Back at the base, the Dalek Section Leader has developed a colony of bacteria which, when released, will wipe out all organic life on Spiridon. All that remains is to immunise the Daleks and the Spiridon slave workers from its effects. Wester arrives at the Thal camp and tells them about the bacteria. The group devises a plan to infiltrate the base by disguising themselves as Spiridons--easy enough to do, as the otherwise-invisible Spiridons wear purple furs that cover them from head to toe. However, they'll also need a Dalek shell for the necessary escort. Having obtained a couple of furs, Taron and Codal join a Dalek patrol. Vaber, meanwhile, has been captured by the Spiridons and delivered to the patrol for interrogation. Rather than reveal the location of the remaining Thals, Vaber makes a break for it and is gunned down by the Daleks. Taron and Codal then retrieve the captured explosives and return to camp. Jo and Latep then lure two Daleks into an ambush at a lake of molten ice; the shock of the lake temperature is enough to kill the Daleks, giving the group the last piece of the plan.