A catastrophic subway accident deep beneath future Tokyo's labyrinthine underground tunnels unleashes something that was never meant to escape containment. What emerges is a symbiotic organism capable of consuming human beings whole—an entity that a secretive cult called Dark Messiah had been studying in the subterranean darkness for years. A lone young man and a frightened schoolgirl named Naomi Sugiura survive the initial carnage and must navigate the warren of tunnels, sewers, and abandoned stations below the city while the creature hunts them both.