For many years, grotesque, Godzilla-like monsters known as “kaijuu” have been sighted throughout Japan. To combat these beasts, an elite military unit known as the Defense Corps puts their lives on the line every day to protect civilians. When a creature is killed, “sweepers” from the Professional Kaijuu Cleaner Corporation are dispatched to dispose of its remains.
Kafka Hibino, 32, is dissatisfied with his job as a sweeper. He has always wanted to join the Defense Corps and kill kaijuus for a living. After a few failed attempts, he gave up on his dreams and resigned himself to mediocrity for the sake of a decent paycheck. Nonetheless, when an ambitious 18-year-old recruit named Leno Ichikawa joins his cleaning crew, Kafka is reminded of his desire to join the military once more.
Following a series of unfortunate events and an encounter with the junior sweeper, Kafka is attacked by a parasite-type kaijuu that forces its way into his mouth, transforming him into a humanoid monster. Kafka intends to use his newfound abilities to give his lifelong dream one last shot.