A year after the death of their young son, struggling writer Seiichi Kirishima brings his wife Miki to a remote tropical island for a vacation meant to repair their fractured marriage. Behind his grieving and apologetic demeanor, however, lies a very different plan: tonight, he intends to kill her and start his life over.

The couple becomes trapped on the island during a torrential downpour. As Seiichi moves to execute his plan in the isolated interior, Miki becomes frightened by his erratic behavior and flees from him. She takes refuge inside the mouth of a dark cave, believing it to be a hiding place from her husband's sudden murderous rage. Seiichi follows her into the darkness, but when he finally catches up to her, his intentions are violently interrupted. Deep within the cave, something else is already waiting. A feral, emaciated old man who has been deprived of sunlight for years lives in the cave's depths, taking women as prisoners and savagely consuming the flesh of any man who wanders into his territory. The hunter quickly becomes the hunted, and Seiichi is forced to flee deeper underground to escape this relentless and inhuman beast.

The story is told largely from Seiichi's perspective, intercut with flashbacks that reveal the events leading to his breakdown. These glimpses into the past show the loss of his writing career after his editor dropped him, the resulting financial despair, and the tragic accidental death of his son which caused his wife Miki to fill with bitterness and blame toward him. As Seiichi struggles for survival in the claustrophobic cave system, the narrative blurs the line between reality and madness, with the sequence of events framed as if it could be the pages of his own final, terrifying novel. The confined setting of the rain-soaked island and the pitch-black, winding cave corridors creates an atmosphere of escalating dread, where the protagonist faces not only a monstrous predator but also the unraveling of his own sanity. The story builds toward an unpredictable and twisted conclusion that reveals the true nature of the horror lurking in the darkness.