Deep in the research wing of the Natural History Museum is a prize specimen, something that comes along much less often than once in a lifetime: a perfect, and perfectly preserved, giant squid. But what does it mean when the creature suddenly and impossibly disappears? For curator Billy Harrow it's the start of a headlong pitch into a London of warring cults, surreal magic, apostates and assassins. It might just be that the creature he's been preserving is more than a biological rarity: there are those who are sure it's a god. A god that someone is hoping will end the world.