Tokiko Mima (nicknamed "Key"), believes herself to be a robot crafted by her scientist grandfather, Murao Mima. Every year, on her birthday, Key believes Dr. Mima builds her a new body, each one size larger. Upon his deathbed, Mima records his will on audio tape, revealing that Key can become human with the aid of 30,000 friends. Key believes she must do this quickly before her battery runs down. The series details the slow unraveling of Key's identity and a secret conspiracy bent on controlling her unique supernatural abilities she develops over the course of the narrative.