Kafka Hibino is a 32-year-old member of the Professional Kaiju Cleanup Corp. His job as a Monster Sweeper is to chop up and dispose of Kaiju killed by the Defense Force. Kafka was also childhood friends with Mina Ashiro who he made a vow with to join the Defense Force together and protect Japan from Kaiju. However, Kafka failed the entrance exam numerous times and quit on his dreams of being a member of the Defense Force while Mina went on to become the captain of the Defense Force’s 3rd Division.
Following a chain of unfortunate events and an interaction with an ambitious recruit named Leno Ichikawa, Kafka encounters a parasite-type kaijuu that forces its way in through his mouth—turning him into a humanoid monster. With his newfound powers, Kafka aims to give his lifelong dream a final try.