Fifty years after a devastating war left Japan in ruins, a new societal order has emerged known as the Sound Health Everlasting Long Life (SHELL) system. By harnessing cutting-edge nanotechnology and healthcare, it promises a life free from disease but demands a grueling 19-hour daily work schedule. However, becoming disconnected from SHELL can trigger the rare "Human Lost" phenomenon, where individuals transform into monstrous beings called "Lost," posing an existential danger to society.
Despite the removal of fear and suffering, antisocial behavior is on the upswing. Takeichi, a factory laborer, decides to assault the Ministry of Interior to protest his oppressive living conditions and persuades his artist friend, Youzou Ooba, to aid him. Their ill-fated mission ends with Takeichi gravely injured and mutating into a Lost, soon followed by Youzou.
Yoshiko Hiiragi, a member of the emergency response team, notices during the clash that Youzou possesses the unusual ability to revert to human form. As the third person to survive after turning into a Lost, Youzou allies with Yoshiko to investigate the rising cases of Human Lost, hoping to preserve what remains of civilization.