Graham Dean inherits a secluded seaside house near San Pedro that once belonged to his great-great-grand-aunt, Morella Godolfo, a woman whispered to have consorted with sorcerers and developed an unnatural affinity for the ocean. Shortly after moving in to paint, Dean begins experiencing persistent, cumulative nightmares involving strange, amorphous sea-creatures and a glowing, emerald underwater realm. Despite warnings from an occultist uncle and a local doctor named Yamada, who explain that ancient sea-dwellers seek to usurp human bodies through a process of mind-transference, Dean remains trapped in the psychological grip of the entity.
The terror escalates when Dean sleepwalks into a coastal cave and shares a blasphemous kiss with an inhuman sea-thing, a meeting that seals his doom. His mind and body are subsequently swapped with the creature, leaving Dean trapped in a grotesque, reptilian marine body while the ancient entity claims his human form. During a harrowing night involving a crashed zeppelin offshore, Dean witnesses the horrific feeding habits of his new companions before returning to the cave. Confronted by his own human body now controlled by the malevolent sea-entity, Dean makes a desperate choice for atonement as his uncle and Doctor Yamada arrive with torches and weapons, culminating in a tragic and startling resolution.