A strange message summons Langdon St. Ives and his companions to the Half Toad Inn in Smithfield, London, and along with the eccentric but fabulously wealthy Gilbert Frobisher, they set out for an uncharted island in the Caribbean, carrying a map that promises a treasure that beggars description. What they find – a terrible, pagan god from the depths of the ocean – leads them back to London, carrying within the hold of Frobisher's steam yacht a fearsome, tentacled menace that threatens to devastate London.