The TARDIS responds to distress signals from no fewer than nine different alien races, all being transmitted from the same part of the planet Ranskoor Av Kolos. There the time travellers meet Paltraki, the lone survivor of a terrible battle. But Paltraki's memory has been shredded by the violent psychotropic waves blanketing the planet. Only the Doctor and her friends can piece together the trail of clues and discover what is happening on Ranskoor Av Kolos. In order to do so, they must confront the mad god who rules that world: none other than Tzim-Sha, the Stenza warrior who killed Grace.