Japan is threatened by monsters on a daily basis. Kafka Hibino, who was aiming to join the "Japan Defense Corps" to defeat monsters, gave up his dream and worked in a cleaning business specializing in monsters. In the midst of his days of feeling anxious about his childhood friend Mina Ashira, who is active as the captain of the 3rd unit of the Defense Corps, and the promise to fight monsters together, Kafka meets Reno Ichikawa, a junior who aspires to be a member of the Defense Forces.