After losing their parents in an accident, Guest also lost their grandfather in a laboratory explosion. Guest, who was visiting the lab, was saved by Rei at the very last second. Rei is a custom-ordered android that the grandfather had built 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 massive inheritance left by the grandfather, there are no financial hardships. Following the grandfather's will, Guest began working at a research facility run 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 sprout within Rei—