Kenji Nishimura — a modern mechanical engineer — is reborn into Sengoku-era Japan in the body of Tsuchiya Sōma, a peasant boy with no status, no martial gift, and no right to become a shinobi. All he has is the knowledge of the twenty-first century. In a world where birthright determines every limit, Sōma uses physics and science to reverse-engineer ninja techniques one principle at a time — and begins climbing toward the summit of a world that was never meant to include him. But the man who understands the rules of a system does not simply learn to survive within it. He learns to break it.