A clandestine conflict rages in Japan’s shadows. A covert organization, National Intelligence of NINJA, or NIN, employs approximately 200,000 ninjas to carry out lethal operations. However, a rival emerges: UNDER NINJA. These ninjas, known as UN, fight NIN at every turn. Kuro, a high school loner dispatched to the front lines, is one such UN. What will become of him, and which group will triumph?