To Every You I've Loved Before

In this world, parallel universes have long ceased to be fantasy: scientists study them, special IP devices track them, and sudden parallel shifts are rare but explainable. Koyomi Takasaki has known this since childhood: after his parents' divorce he stays with his mother, but once he slips into a world where he lives with his father, his grandfather is alive and his beloved dog is dead. Years later, the withdrawn, socially awkward Koyomi enrolls in a prep school and tries to live as if there were only one reality, until his classmate Kazune Takigawa tells him she has come from the 85th world, where the two of them are in love.

Over time their story goes beyond school romance. They grow up, enter university, become linked to the Institute of Imaginary Science and start studying the crossing of lives across worlds. Their love becomes a daily choice: to stay together even when one cannot be sure this is the same version of the person one loved yesterday. Then a more troubling mystery intrudes, as forced shifts to a specific world become possible, threatening not only their place in their own reality but the boundaries of their identities.