Kaiju No. 8 takes place in a world constantly threatened by giant monsters known as Kaiju. Kafka Hibino, once full of ambition, dreamed of joining the prestigious Defense Force alongside his childhood friend, Mina Ashiro, to protect humanity from these monstrous threats. However, as life progressed, Mina rose through the ranks to become a formidable officer in the Defense Force, while Kafka’s path took a different turn. He now works in a cleanup crew, tasked with removing the remains of Kaiju after battles.

Kafka’s mundane life changes when he meets Reno Ichikawa, a determined young man with aspirations of joining the Defense Force. Inspired by Reno’s resolve and reminded of his own forgotten dream, Kafka’s determination to fulfill his promise to Mina is reignited. However, fate has an unexpected twist for Kafka, as a strange incident turns him into something unheard of—a human-Kaiju hybrid. With his newfound abilities, Kafka embarks on a journey to finally join The Defense Force and fight alongside Mina to protect humanity from the Kaiju threat, all while hiding the monstrous power within himself.