The Natsume family ΓÇö Professor Kyusaku, his wife Akiko, and fourteen-year-old son Ryunosuke ΓÇö take in a young woman by the name of Atsuko, who has lost her memory. Unbeknownst to all of them, Atsuko is an Androrobot prototype which has escaped from the power Mishima Corporation's research facilities.
In order to track down the missing prototype, Mishima transferred Akiko, who is employed by the corporation, to a new secret department the hunt for the Androrobot. Professor Kyusaku has secrets of his own and discovers that Atsuko is an Androrobot and sneaks Atsuko down into his hidden lab under the house in order to attempt to unlock her secrets. Ryunosuke knows there's something different about Atsuko but she seems pretty normal outside of the fact that she can't remember anything about herself, and he is quite happy to have her around.
 
http://en.wikipedia.org/wiki/All_Purpose_Cultural_Cat-Girl_Nuku_Nuku_DASH!