A man working a job far removed from his childhood dreams gets wrapped up in an unexpected situation - Becoming a monster, he aims once again to fulfill his lifelong dream.

In Japan, monsters known as kaiju regularly attack the populace with the Japanese Defense Force tasked with killing them. After their town was destroyed by kaiju when they were children, childhood friends Kafka Hibino and Mina Ashiro both vowed to become members of the Defense Force. Mina has become famous as commander of the Defense Force's Third Unit, but Kafka has failed the examination numerous times and is a member of the clean up crew, Monster Sweeper Inc., whose job is to dispose of the monsters' dead bodies after battle. After a small talking monster flies into his body via his mouth, Kafka gains the ability to turn into a monster himself, which gets dubbed "Kaiju No. 8" by the Defense Force. Kafka remains fully cognizant while in the form, but gains superhuman strength, and becomes the first monster to escape the Defense Force. With newfound resolve, Kafka decides he wants to fight by Mina's side and finally joins the Defence Force while also keeping his new abilities a secret.