When Ryunoskue's cat dies in an accident, his inventor father resurrects the cat by placing its brain in an android body resembling a young girl. The android, Nuku Nuku has the role of companion/sister for Ryunosuke as well as bodyguard for his father. Nuku Nuku's extreme strength and cat-like agility are regularly needed to protect Ryunosuke and his father from Ryunosuke's mother who seeks to kidnap them following her divorce, Ryunosuke out of maternal love and her ex-husband to force him to create weapons of mass destruction for his arms company.