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.

The Natsume family moves back together in the Mishima Manson under the condition that Akiko takes on all the duties of a regular mother and wife. She discovers that because of all the work she has to do she hardly sees Ryunosuke any more than before, so she plots revenge on Kyusaku for 'tricking' her.

A young girl named Eimi is found in the remains of a crashed ship after escaping from Mishima Industries.

Nuku gets a job at a local restaurant to pay for a new bike for Ryunosuke.

A new Mishima satellite weapon has accidently been set to kill Akiko, So she enlists the help to Nuku and Eimi to disable the satellite.