When Ryunosuke's pet cat dies in an accident, his inventor father "resurrects" the cat by placing its brain in an android body made in the form of a young girl. The Android, Nukunuku, has the dual purposes of being a companion/sister for Ryunosuke as well as being a bodyguard for his father. Nukunuku's extreme strength and feline agility are regularly needed to protect Ryunosuke and his father from Ryunusuke's mother who seeks to kidnap them both following her divorce from his father. Her motives towards Ryunusuke are simple motherly love, however her genius-inventor-ex-husband she wants to put to work designing weapons of mass destruction for her weapons company.

[Source: Anime News Network]

Each episode centers on Nuku Nuku's adventures and mishaps in high school, and her relationship to students, the overall series plot centers on the evil Mishima Industries' scheme to dominate the world through its products. In opposition to this plan are the slightly nutty Kyuusaku Natsume and Nuku Nuku, the android who acts as a companion to his young son. Nuku Nuku's classmates are all eccentrics, Futaba, the class president, is a domineering control-freak, who cannot leave her authority in school. Chieko Shirakaba is the rich snob with the two yes-girls who always take her side. There is a bookworm, a mystic, a scientist, and a pop singer. All of these are true to their descriptions: the mystic is always foretelling doom via tarot cards, spirits, or some other occult technique. The pop singer never says anything except as a song. The bookworm always has her nose in a book. These eccentricities add to the series' humor.

[Source: Anime News Network]

A strange girl, Atsuko Higuchi, shows up at Ryunosuke's house and tells him to call her "Nuku Nuku." Her tragic past has left her with amnesia... but more importantly, he's head over heels for her!

[Source: Crunchyroll]