The setting is the sleepy lighthouse town of Piedras Blancas. Sturges (Harmon) is the lighthouse keeper of the town and is very superstitious and concerned for the safety of his teenage daughter, Lucy (Carmen). He leaves food for a sea monster who lives in a nearby cave. The locals disregard him at first, but they begin to take notice when the bodies of people killed by the monster are found on the beach. A local scientist identifies a scale found near one of the bodies as belonging to a "diplovertebron", a prehistoric amphibious reptile long presumed extinct.