The plot of the book revolves around an eponymous document which describes a supernatural entity, the Eater of Souls. The document and Howard's boss James Angleton go missing, and Howard must locate them. Angleton turns out to be involved in a struggle with cultist double agents inside the Laundry loyal to Nyarlathotep, who capture Howard and plan to bind the Eater of Souls into Howard's body in order to advance their goals. This fails because the Eater of Souls was already bound into Angleton's body decades ago by the predecessors of the Laundry; it has "gone native", aligning itself with the Laundry's goals and British values. Howard uses magic to raise the dead, using them to overcome the cultists.