In Japan, a "monster country", people's daily life is ruthlessly damaged by monsters. The protagonist Hibino Kafka, the city where he lived when he was a child was destroyed, and he made an agreement with his childhood friend Abai Mina to "wipe out all the monsters". However, the 32-year-old Kafka was disillusioned and worked for Monster cleaning Company, a company that disintegrated monster corpses. Compared with Mina, who became increasingly active as a member of the Japanese Defense Force, Kafka led a melancholy life every day. Ichikawa Leno, who came to work, told him that the age limit for the defense force would be raised and advised him to take the team entrance exam. So Kafka, who made up his mind again, was suddenly eroded by mysterious creatures, and his body began to become a monster, known as "Monster 8".