The TARDIS is summoned to a space cathedral by Shadrak Winter, the High Cardinal of the Cult of the Prime Self. Soon to be relieved of his post by way of assassination, Shadrak has used his family’s calling card to bring the Doctor to his aid. His followers have long fled the Sinful, highly contagious slug-like creatures which he himself has succumbed to—and which now imperil the Doctor and Clara.

As Clara discovers more about the Cult of the Prime Self — currently the fourth most popular religion — the Doctor becomes infected, all but lost to appalling guilt and self-recrimination. As he seeks to absolve himself of his perceived sins, Clara must use all her survival instincts to escape the Sinful’s advances and rescue her vulnerable friend.