"Kaiju No. 8" is set in a world where humanity is under constant threat from monstrous creatures known as Kaiju. To combat this menace, the Japanese Defense Force (JDF) employs specialized units tasked with neutralizing these threats. Kafka Hibino, a 32-year-old man working in a Kaiju cleanup crew, harbors a lifelong dream of joining the Defense Force. However, his age and perceived lack of talent have kept him from achieving this goal. His childhood friend, Mina Ashiro, has risen to the rank of captain within the Defense Force, further fueling Kafka's determination to prove himself.

The narrative takes a dramatic turn when Kafka is infected by a parasitic Kaiju, transforming him into a humanoid Kaiju with immense strength and regenerative abilities. Despite his new form, Kafka retains his human consciousness and resolves to use his newfound powers to protect humanity. Adopting the alias "Kaiju No. 8," Kafka secretly joins the Defense Force's recruitment program, aiming to fulfill his dream while concealing his true identity.

The series explores Kafka's dual existence as both a Kaiju and a human, navigating the challenges of maintaining his secret while proving his worth to the Defense Force. Alongside Kafka, the story introduces a diverse cast of characters, including Reno Ichikawa, a young recruit who becomes Kafka's ally, and Kikoru Shinomiya, a prodigious cadet with a complicated relationship with her father, a high-ranking officer in the Defense Force. The dynamics between these characters, as well as their individual struggles and growth, are central to the narrative.

As Kafka and his comrades face increasingly dangerous Kaiju threats, the series delves into themes of identity, perseverance, and the moral complexities of wielding power. The Defense Force's internal politics and the public's perception of Kaiju also play significant roles in shaping the story. Tensions rise as Kafka's secret becomes increasingly difficult to conceal, and the line between humanity and monstrosity blurs.

The plot is driven by a mix of intense action sequences, character-driven drama, and strategic battles against Kaiju, culminating in high-stakes confrontations that test the limits of Kafka's abilities and resolve. The series balances its exploration of Kafka's personal journey with the broader conflict between humanity and the Kaiju, creating a compelling narrative that intertwines individual struggles with larger societal challenges.