In Japan, a "monster country", people's daily lives are ruthlessly destroyed by monsters. The protagonist Kafka Hibino, whose city was destroyed when he was a child, made a promise with his childhood sweetheart Mina Ashiro to "destroy all the monsters." However, at the age of 32, Kafka's dream was shattered and he worked at a monster corpse dismantling company called "Monster Cleaning Company." Compared to Mina's increasingly active role as a member of the Japanese Defense Force, Kafka lived a melancholy life every day. Ichikawa Reno, who came to work part-time, told him that the age limit of the Defense Force would be raised and advised him to take the entrance examination. Kafka, who made up his mind again, was suddenly invaded by a mysterious creature and his body began to become a monster, and he was called "Monster No. 8."