Kafka and his childhood friend Mina had vowed to kill monsters (Kaiju) when they grew up. But as fate would have it, Mina became a high-ranking officer, while poor 32-year-old Kafka is working a job cleaning up the garbage of dead monsters. Then he meets a new friend, Reno, who reminds him of his old dream.
One day, a monster attacks both of them. Kafka saves it but ends up in the hospital himself. Suddenly, a small insect enters him, and Kafka himself becomes a monster—but his mind remains human. The army has now named him “Kaijū No. 8.” Kafka decides to use his powers for good and, along with Reno, attempts to rejoin the army so he can be with Mina.
