"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 Japan Defense Force (JDF) operates specialized units tasked with neutralizing these threats. The story centers on Kafka Hibino, a 32-year-old man who works in a kaiju cleanup crew, disposing of the remains of defeated kaiju. Despite his mundane job, Kafka harbors a lifelong dream of joining the Defense Force to protect humanity, a dream he once shared with his childhood friend, Mina Ashiro, who has since risen to the rank of captain in the JDF.

Kafka's life takes a dramatic turn when a small, talking kaiju infiltrates his body, transforming him into a human-kaiju hybrid. This transformation grants him immense strength and regenerative abilities but also makes him a target for the very organization he aspires to join. Now designated as "Kaiju No. 8," Kafka must conceal his identity while striving to achieve his dream of joining the Defense Force. Alongside him is Reno Ichikawa, a young and determined recruit who shares Kafka's ambition and becomes his ally in navigating the challenges of the JDF's rigorous training program.

The narrative explores Kafka's dual existence as both a potential savior and a perceived threat. As he progresses through the Defense Force's ranks, he faces internal and external conflicts, including the constant risk of exposure and the moral dilemmas of wielding his newfound powers. Mina Ashiro, now a high-ranking officer, remains a central figure in Kafka's journey, embodying the ideals he strives to uphold while also representing the organization that could ultimately see him as an enemy.

The series delves into themes of perseverance, identity, and the blurred lines between humanity and monstrosity. It balances intense action sequences with moments of introspection, as Kafka and his comrades confront not only the physical threats posed by kaiju but also the ethical and emotional challenges of their roles. The dynamic between Kafka, Reno, and other recruits adds depth to the story, highlighting camaraderie and rivalry within the Defense Force. As the plot unfolds, the stakes escalate, with Kafka's hybrid nature becoming both a critical asset and a source of tension in the ongoing battle against kaiju.