London, 1893. When Cora Seaborne's controlling husband dies, she steps into her new life and, along with her son Francis, she leaves town for Essex in the hope that fresh air and open space will provide refuge.
On arrival, rumours reach them 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 local people 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.
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.