In 2143, humanity has lost the ability to reproduce after a comet-induced genetic mutation. Shinonome Kagami, an apprentice in the craft of creating homunculi, meets Shirayuki, a girl who resembles his deceased childhood friend, and is drawn into secrets that concern the survival of civilization.

In a world where humans take on the divine task of creating life, a boy and a girl's meeting sets the future in motion.

High school student Gudo Shiraishi remembers a former life in which he saved humanity. When Mai Shiina transfers into his class, he learns that she too is a reincarnator: the witch he once fought in another world. Their present-day school life becomes entangled with the history they share from their previous lives.

A coming-of-age romantic comedy in which a hero and a witch, once enemies in another life, meet again at a modern high school.