In the tranquil Japanese countryside lives Tokiko Mima, known as 'Key,' a 17-year-old girl who is actually a robot in human form. After her grandfather, Dr. Murao Mima, passes away, he reveals a secret message to Key: she can achieve humanity by making thirty thousand friends. Determined, Key leaves her serene home in Mamio Valley for the bustling city of Tokyo, where she reunites with her childhood friend Sakura Kuriyagawa. Intrigued by the popular idol singer Miho Utsuse, Key ponders if a career in music could help her amass the required friends. However, Miho harbors a dark connection to Jinsaku Ajou, an adversary of Dr. Mima seeking advancements in robotic weaponry. As Key navigates the path to becoming human, Ajou schemes in the shadows, unveiling Key's extraordinary past and setting a perilous chain of events in motion.