"Under Ninja" is a manga series that explores the clandestine world of modern-day ninjas operating in contemporary Japan. The narrative centers on Kuro Kumogakure, a high school student who is also a member of an underground ninja organization. This organization, known as the NIN, operates in secrecy, carrying out missions that range from espionage to assassination, all while maintaining the facade of ordinary civilian life. The story delves into the duality of Kuro's existence, balancing his mundane school life with the dangerous and morally complex tasks assigned by the NIN.

Kuro is not alone in this dual life; he is part of a larger network of ninjas who are similarly embedded in society. The organization is highly structured, with a hierarchy that dictates missions and enforces strict codes of conduct. The series introduces various characters within the NIN, each with their own unique skills and backgrounds, contributing to the intricate dynamics of the group. Among these characters are Kuro's peers, mentors, and rivals, all of whom play significant roles in the unfolding plot.

The missions undertaken by Kuro and his fellow ninjas often involve high-stakes scenarios, including political intrigue, corporate espionage, and personal vendettas. These missions are not only physically demanding but also morally challenging, forcing the characters to confront the ethical implications of their actions. The series frequently explores themes of loyalty, identity, and the cost of living a double life.

As the story progresses, Kuro begins to uncover deeper layers of the NIN's operations, including hidden agendas and internal conflicts. These revelations complicate his understanding of the organization and his place within it. The narrative builds tension through a series of escalating missions and personal trials, culminating in a broader exploration of the ninja's role in a modern world that has ostensibly moved beyond their traditional functions.

"Under Ninja" combines action, suspense, and character-driven drama, presenting a nuanced portrayal of ninjas in a contemporary setting. The series maintains a focus on the interplay between the characters' personal lives and their professional duties, creating a rich and engaging narrative that appeals to fans of both action and psychological storytelling.