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.