London 1893. When Cora Seaborne's controlling husband dies, she steps into her new life as a widow with as much relief as sadness. Retreating to the countryside with her son, she encounters rumours of the 'Essex Serpent', a creature of folklore said to have returned to roam the marshes. Cora is enthralled, believing it may be an undiscovered species. Setting out on its trail, she collides with local minister William Ransome, who thinks the cure for hysteria lies in faith, while Cora is convinced that science offers the answers. Despite disagreeing on everything, he and Cora find themselves drawn together, changing each other's lives in unexpected ways.