London, 1893. When Cora Seaborne's controlling husband dies, she steps into her new life as a widow with as much relief as sadness. Along with her son Francis - a curious, obsessive boy - she leaves town for Essex, in the hope that fresh air and open space will provide refuge.

On arrival, they hear rumours that the mythical Essex Serpent, once said to roam the marshes claiming lives, has returned to the coastal parish of Aldwinter. Cora, a keen amateur naturalist, is enthralled, convinced that what the locals think is a magical beast may be a yet-undiscovered species. As she sets out on its trail, she is introduced to William Ransome, Aldwinter's vicar, who is also deeply suspicious of the rumours, but thinks they are a distraction from true faith.

As he tries to calm his parishioners, Will and Cora strike up an intense relationship, and although they agree on absolutely nothing, they find themselves at once drawn together and torn apart, affecting each other in ways that surprise them both.