Yuko suggests throwing a surprise birthday party for Nano, which Mai and Mio agree with. In the Shinonome Laboratory, the Professor has made a smaller wind-up key for Nano, only for Sakamoto to accidentally crush it. The Professor promises to have a new one ready before Nano gets home from school. At school, the Principal is shocked to see his statue replaced by one of the Vice-Principal, whose plot is exposed when the movers reveal they still have the Principal's statue and dislike the Vice-Principal's statue. After Yuko avoids several accidents with her friends and unintentionally helps others, she tells Nano that she, Mai and Mio will be coming to her home as the three are holding the surprise party there. Ms. Nakamura finally finds her pet cat Taisho, who is actually Sakamoto. Sakamoto is horrified to see his old owner due to how badly she took care of him, and manages to escape while Nakamura is asking Nano to find and return Taisho if she ever finds him. When Nano returns home, the Professor removes Nano's wind-up key, revealing that anyone except Nano herself could have removed the key at any time. As Yuko, Mai and Mio arrive, Nano, after some thought, decides to keep the wind-up key after realising that she already has a normal life, and none of her friends care that she's a robot. As everyone celebrates Nano's birthday, Nano learns to her chagrin that the smaller wind-up key was actually an add-on that makes her wind-up key spin when she's happy. Despite that and with two birthday cakes ruined by the excited Professor, everyone has a good time at the party, and Nano's key spins.