On a Bermuda beach, Magnus Dens, Leigh McCloskey, is approached by a beautiful but mysterious woman, Jennie Haniver Connie Sellecca, who seems to recognize him. He slips into childhood memories of that same shoreline, where he and Jennie played as friends, watching a turtle egg hatch. The scene expands to years later, with the two of them as youths playing with a growing turtle. Magnus carves "J+M" inside a heart on the shell, while Jennie crafts a cowrie shell necklace for him. The memory shifts again to a younger Magnus watching Jennie ride the turtle toward the open sea, only to vanish beneath the water as he calls after her. Suddenly, the dream dissolves into a nightmare of the night his father, Lionel, was knocked into the water by an unseen horror in the cave beneath the house, the cliffside dwelling shaking and crumbling as a storm rages.
When Magnus awakens, he roams to town and meets his old friend Eric. Eric introduces him to Dr. Paulis, who is pursuing teratology as part of his marine biology work. Their reunion is interrupted when the boat listing violently to one side reveals that the net they pulled up has been shredded by something enormous and powerful.
That evening, Doshan, Julie Woodson, joins Eric for dinner at Dr. Paulis’ house. Magnus is drawn to Delia, Ruth Attaway, and the necklace she wears—one that resembles his own. The conversation does little to answer his questions about his father’s death, and he steps outside, where he again glimpses Jennie swimming near the boat. He dives in to follow her but is saved from drowning by the mysterious woman, who returns to the sea and introduces herself as Jennie Haniver. Dr. Paulis dismisses his tale, suggesting Jen-nie Haniver may be a local legend or a prank. Delia, however, later tells him the full legend: Jennie Haniver was a beautiful but vain woman who bargained with a god to survive a violent sea storm and was granted eternal life, only to become a sea creature doomed to live in the water and never on land.
The next morning, the police bring Eric to examine the enormous tracks on the beach. Magnus encounters Jennie again and welcomes her back into his memories as she describes the quadrilles her father would hold in their great hall. Eric calls Magnus away to join him and Dr. Paulis on a new expedition. The trio sets out again, and once more the boat is forced to list as something unseen tugs at their nets.
That night, Magnus confronts Dr. Paulis about his father’s death. Paulis explains Lionel’s work—mutations in sea life—before Magnus asks if he remembers the turtle from childhood, recalling the initials he carved for himself and Jennie. Paulis reacts with dismay to the memory.
Soon after, Eric and Paulis argue about how to proceed. Eric wants to unleash the Horror, a harpoon-firing bazooka, while Paulis recoils at the idea. Despite Paulis’ objections, Eric gathers Magnus and heads out to sea. They resume trawling, and the line begins to drag. Eric dives to save the net and shoots at a form he can barely see; it is Jennie. The two men continue, and the enormous turtle—so large it tears through the net—emerges into view. Eric blasts it with the Horror and lets the creature run out the line, waiting for it to resurface.
After sunset, the boat’s power fails and the compass spins wildly. Magnus steps out of the cabin and finds Jennie on deck, begging him to release the turtle, hinting that she made a promise long ago that she now regrets. At Doshan’s insistence, Dr. Paulis flies out in a helicopter to reach the boat. He witnesses a vast shape moving underwater near the vessel, and the turtle surfaces, leaping toward the helicopter and sending it crashing into the ocean. Eric rushes onto the deck but is confronted by Jennie; her eyes glow a pale green as the turtle surfaces nearby, dragging Eric helplessly into the depths.
Jennie returns to Magnus, kissing him as he lies unconscious on the beach, then disappears back into the sea for the last time. Doshan confronts Magnus in the graveyard where his parents lie, sharing her horror at the losses they’ve suffered. He tells her he plans to leave Bermuda and cut ties with the sea. He walks away, not noticing the gravestone sculpture of Jennie Haniver and the inscription reading “Jennie Haniver, 1701- , Lost at Sea.”
On the ferry leaving Bermuda, Magnus removes the necklace Jennie had given him and drops it into the waves. It sinks past the great turtle, which surfaces in the distance with the same initials Magnus carved on its shell so many years ago, symbolizing a bond that cannot be fully severed, even as he tries to let go.
