The series is based on the eponymous novel by Sarah Perry. The story revolves around a young Englishwoman named Cora Seaborne. After burying her husband at a young age, she leaves London and travels to Essex, drawn by the legend of a mythical serpent that is said to have appeared there. In Essex, Cora befriends a village pastor. Soon, tragic events occur, and the townspeople accuse the outsider. Now, Cora must not only unravel the mystery of the mystical serpent but also prove her innocence.