Thomas Hart and Grace Macaulay have lived all their lives in the small Essex town of Aldleigh. Though separated in age by three decades, the pair are kindred spirits --- torn between their commitment to religion and their desire to explore the world beyond their small Baptist community. It is two romantic relationships that will rend their friendship, and in the wake of this rupture, Thomas develops an obsession with a vanished 19th-century astronomer said to haunt a nearby manor, and Grace flees Aldleigh entirely for London. Over the course of 20 years, by coincidence and design, Thomas and Grace will find their lives brought back into orbit as the mystery of the vanished astronomer unfolds into a devastating tale of love and scientific pursuit.

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.