Imp of the Perverse is set in a Gothic Jacksonian America of the 1830s-40s where monsters are real. One player takes on the role of the Editor, who creates monsters and sets the stage for the rest of the players, who take on period-appropriate protagonists, to fight against the horrors that stalk the night. These protagonists have an Imp of the Perverse on their own shoulders, impelling them to commit terrible deeds. Only by fighting those who have given in to their own Imps, and literally transformed into creatures of the night, can they gain redemption and reject their Imp - but the struggle itself continually tempts them to embrace their perversity and become damned themselves.
This world contains a Shroud between the world of the living and the world of the dead. Imps come from beyond the Shroud, and they actively want to drag their afflicted back across. Hence, whenever a monster appears, all those with Imps on their shoulders can feel it, like a pressure wave of perversity sweeping over them. Every Chapter starts with the protagonists actively aware that something is wrong and a monster is near; gameplay is about discovering the specific nature of the creature, exploring how the horror it brings to the world resonates with your perversity, and then confronting the creature to end the threat it poses.