London 1893: When Cora Seaborne and her son Francis reach Essex, rumours spread from further up the estuary 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 it may be a previously undiscovered species. As she sets out on its trail she meets William Ransome, Aldwinter's vicar. They find themselves inexorably drawn together and torn apart, eventually changing each other's lives in ways entirely unexpected.