When Cora Seaborne's domineering husband dies, she steps into her new life as a widow with as much relief as sadness. Seeking refuge, Cora leaves London for a visit to coastal Essex, accompanied by her son, Francis, and the boy's nanny, Martha, her fiercely protective friend. Cora learns of a fearsome creature said to roam the marshes claiming human lives. After nearly 300 years, the mythical Essex Serpent is said to have taken the life of a young man on New Year's Eve. Certain that the "sea beast" may be a previously undiscovered species, Cora joins local vicar William Ransome in investigating the rumors.