From the introduction:
This is a game about four cultists working in the shadows to bring about the end of the world. The Grand Convergence – an alignment of heavenly bodies that only occurs once a millennium – approaches, and with it a rare opportunity to call forth an otherworldly entity of unimaginable power into the Earthly plane. Extradimensional portals don't just open themselves, though. It's going to take a lot of effort on the cultists' part to drown the world in blood and darkness.
However, all is not what it seems. At the eleventh hour, one of the cultists is revealed to be an investigator in disguise, working from within to foil the cult's plans. But will it be enough to save the world? Or will the cultists make good on this last chance to summon their dark god?