Veteran epidemiologist Carla Edwards has been dispatched by the C.D.C. to investigate a cluster of inexplicable mutations among the young people of Innsmouth, a sickly and destitute town on the Massachusetts coast. Initially skeptical, she rapidly discovers that the true mystery is older and more horrifying than anything for which her training has prepared her. As the danger mounts, a double helix of history and urban folklore draws her inexorably to the door of a sinister, evangelical cult – and beyond the limits of her science and belief.