Female mutants (new humans or superpowers) with cat-like horns on both sides of their heads are called dicynodonts. Because they have the power to destroy humanity, the National Institute of Ecology, located on an outlying island off the coast of Kamakura City, Kanagawa Prefecture, Japan, has secretly isolated these girls for research. One day, a major accident occurred in the institute. Lucy, a dicynodont girl who was originally isolated, was able to break through the restraints due to the negligence of the guards and killed the armed guards in the institute. Even Kisaragi, the secretary of the director of the institute, Kurama, was killed. Although the sniper aimed at her head with an armor-piercing bullet and shot, he only broke her iron helmet; although Lucy was injured in the head and bleeding and fell into the sea, she still survived and successfully escaped. The day after the incident, Kota, a young man who went to Kamakura to ask his relatives for help in order to go to college, reunited with his childhood playmate, Yuka, a cousin. They traveled together to the seaside full of memories, and met Lucy, who had been replaced by another personality due to a head injury; she could only make the sound of Nyu, so she was named Nyu. Nyu had no idea that she was the existence that caused the extinction of mankind, so Kota and Yuka decided to take her to Fengzhuang where they lived to settle temporarily. At this time, the institute dispatched SAT (Special Assault Team) to hunt down the escaped Lucy, and SAT member Bando found Nyu with a changed personality on the coast. The mission was about to succeed, but because Nyu restored Lucy's personality, all the members traveling with Bando were killed, leaving only Bando who was seriously injured (lost his right arm and both eyes) after the defeat. He vowed to avenge Lucy, and resurrected with the help of the institute's science and technology. Later, he escaped from the institute, left SAT and returned to Kamakura, searching for Lucy's whereabouts by any means...