The central character of the story is Mima Tokiko (aka "Key"). Key is a robot, crafted by her scientist "grandfather", Mima Murao. Every year, on her birthday, "grandpa" builds Key a new body--one size larger. Upon his deathbed, Murao reveals that Key no longer has to remain a robot: she can become human with the aid of 30,000 friends. Key must do this quickly before her battery runs down. However, some very powerful men are after Key and the secrets her body contains.