Peter Leland, a scholarly Methodist minister, inherits an old family home and moves in with his wife for an extended sabbatical. He is working on a book about the relation between the ancient worship of the Philistine god Dagon and modern obsession with sexuality that pervaded culture (for context the book was published in 1968). Walking around in it he begins to become obsessed with the place and with the papers of his grandfather (or great-grandfather) who seemed to be involved in a pagan religion of some kind; the reader of Lovecraft will recognize it as some splinter of the Cthulhu cult. It becomes likely that the Biblical Dagon and the lovecraftian Dagon are the same entity. He and his wife encounter a family that live on his land. They claim to have done so for generations. There is something off about them; the reader of Lovecraft will recognize that they seem to come from a similar lineage to the fish people in Shadow over Innsmouth. Peter begins to become obsessed with the daughter of that family. From there the Southern Gothic and Lovecraftian tropes mix in fascinating ways as Peter loses his grip on reality, does horrible things and has horrible things done to him.
This is a brilliantly conceived, well executed, and extremely disturbing story that I suspect will stick with me much longer this time.