In late 19th century Essex, newly widowed Cora Seaborne learns of a rumor of a creature said to roam the marshes claiming human lives. After nearly 300 years, the mythical Essex Serpent is said to have returned, taking the life of a young man on New Year’s Eve. Cora is immediately enthralled, and certain that what the local people think is a magical sea beast may be a previously undiscovered species. Eager to investigate, she is introduced to local vicar William Ransome who is also suspicious of the rumors but believes they have a different cause.