I own the ADV version of the OVA as well as the four DVDs for the TV series, and I can say the OVA is the superior Nuku Nuku. The plot is after a cat is gunned down in an attack targeting scientist Kyusaku, who is on the run with his son Ryunousuke after stealing a prototype android he made to prevent it from being sold as a weapon. Seeing poor Ryunouske in tears, Kyusaku decides to give him the best Christmas present by transferring the cat's brain into the android body he stole, and makes it into the shape of a magenta-haired teenage girl, Nuku Nuku is born.
So basically, the first three episodes revolve around Akiko's plot to get back Ryunosuke, which are thwarted by Nuku Nuku herself. The fourth episode introduces Eimi, an android with a defective body who want to take Nuku's body for her own, which causes trouble for the heroine, even coming back to fight her in the sixth and final episode. While the first three are good, the final three are kind of so-so. However, the series does well. While it is a comedy, it also doubles as action, as Nuku Nuku has to fight to protect her Papa San and Ryunosuke from Akiko's henchwomen, all while trying to be human, as she acts like a cat, which can lead to comedic situations. This is an anime I recommend watching.