After losing their parents in an accident and then their grandfather in a lab explosion, Guest was left alone. Guest, who was visiting the lab, was rescued by Rei at the very last second. Rei is an android the grandfather specially commissioned and left behind for Guest.

Thanks to the grandfather leaving Rei behind, Guest was spared from being completely alone in the world. Furthermore, thanks to the vast inheritance left by the grandfather, there are no financial struggles. Following the grandfather's will, Guest began working at a research lab owned by an acquaintance of his.

Rei supports Guest devotedly and watches over them constantly. However, at some point, he begins to ignore his programming. A sense of self begins to awaken within Rei—