One hot summer’s day, John Cole decides to shut his bookshop early, and possibly forever, and drives out of London to see his brother. When his car breaks down on an isolated road, he goes looking for help and finds a dilapidated house. As he approaches, a laughing woman he’s never seen before walks out, addresses him by name and explains she’s been waiting for him. Entering the home, John discovers an enigmatic clan of residents, all of whom seem to know who he is, and also claim they have been awaiting his arrival. They seem to be waiting for something else, too --- something final.

It has been years since Helen Franklin left England. In Prague, working as a translator, she has found a home of sorts --- or, at least, refuge. That changes when her friend Karel discovers a mysterious letter in the library, a strange confession and a curious warning that speaks of Melmoth the Witness, a dark legend found in obscure fairy tales and antique village lore. As such superstition has it, Melmoth travels through the ages, dooming those she persuades to join her to a damnation of timeless, itinerant solitude. To Helen, it all seems the stuff of unenlightened fantasy. But, unaware, as she wanders the cobblestone streets, Helen is being watched. And then Karel disappears.

In the wake of her husband's death, Cora Seaborne leaves London for a visit to coastal Essex. There, she learns of an intriguing rumor of a fearsome creature said to roam the marshes claiming human lives. After nearly 300 years, the mythical Essex Serpent is said to have returned, taking the life of a young man on New Year’s Eve. Cora is certain that what the local people think is a magical sea beast may be a previously undiscovered species. Eager to investigate, she is introduced to local vicar William Ransome, who is convinced that the rumors are caused by moral panic, a flight from true belief. These seeming opposites soon find themselves inexorably drawn together and torn apart --- an intense relationship that will change both of their lives in ways entirely unexpected.