[Based off the song Kokoro by Rin Kagamine, which can be found here.] 

Kagamine Len is a lonely scientist living by himself when he finally succeeds in creating a robot to which he named Rin, after his deceased twin sister. Rin, still very new to the world, spends her first few days living with Len, who confides in her and takes care of her. In order to make her feel human "emotions", Len designs a programme "Heart". Rin, meanwhile, decides to devote her time to singing in order to fully cherish the time that she and Len spent together.