Natsume Akiko is the female president of Mishima Corporation. One day, her husband, Natsume Kyūsaku—a researcher at Mishima Corporation—discovers that the humanoid robot he is developing will be used as a weapon by the chairman of the Mishima conglomerate (Akiko’s grandfather). He flees with the prototype robot NK-1124 and their beloved son, Natsume Ryūnosuke. Enraged by the abduction of her son, Akiko orders Kyōko and Arisa-chan to retrieve Ryūnosuke. Later, on Christmas Eve, while Ryūnosuke argues with Kyūsaku over wanting to adopt a stray cat, they are discovered by the pursuers sent by Akiko—Kyōko and Arisa—and come under attack from the new-model helicopter Poison1. In the chaos, the cat Ryūnosuke picked up is hit by a stray bullet. Seeing Ryūnosuke’s deep sorrow and moved by pity for the cat, Kyūsaku transplants the cat’s brain into NK-1124. Thus begins the story of Nuku-Nuku, revived as an all-purpose cat-brain robot, and the people around her.