After losing their parents in an accident and their grandfather in a laboratory explosion, Guest was saved at the last moment by Rei. Rei is an android that the grandfather had custom-made and left behind specifically 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 institute run by his acquaintance.

Rei supports Guest with total devotion, watching over them constantly. However, at some point, he begins to ignore his programming. A sense of self begins to sprout within Rei—