The corpse of an unknown creature washes up on the coast of the Pacific Ocean. It is gigantic and serpentine, with a head that appears to be a mass of tumors and small tendrils. Scientists want to preserve it, believing it to be a prehistoric organism, but it is already beginning to rot. They also observe that patches of its skin are transparent.
Among the crowd of people who have come to see the discovery is a boy who is terrified of fish and the ocean in general, fearing the mysteries that could be lurking in its unfathomable depths. He has suffered from recurring nightmares of floating in the sea, surrounded by giant, grotesque species unlike anything known to exist. Despite this, he feels compelled to see the creature with his own eyes.
As the scientists continue to examine the creature, the boy strikes up a conversation with a young woman named Mie. Her fiancé Tadashi was lost at sea many years ago; he was on a ferry which sank, with none of the people on board ever found. Some time ago, Mie has started dreaming that she is Tadashi, floating next to people she doesn't notice; terrifying sea creatures surround her, kept at bay by an invisible wall. The boy realizes this is the exact same dream he has had.
Meanwhile, the people looking at the creature notice something is still moving inside it. Peering through the transparent patches of skin, they discover what appear to be hundreds and hundreds of people trapped inside the creature. Although they are in what should be its intestine, they haven't been digested. The creature's stomach is cut open to release the people, revealed to be everyone who disappeared in the ferry crash, including Tadashi. Although they are still alive, they have become zombie-like parasites living off the creature, and all are declared insane. The boy, remembering his dream, wonders what horrors they could have seen in the sea, through the creature's transparent skin.