In Hidaka Koyomi’s world, travel between slightly different alternate universes is commonplace. After his parents’ divorce, Hidaka now lives with his father. One day, he falls in love with a girl named Satou Shiori after meeting her at his father’s workplace. Unfortunately, their young love seems doomed as their parents are getting remarried…to each other. Is there some parallel world out there where these two can be together as lovers, not step-siblings…and what could be the price if they try to find out? and are parallel universe novels by the same author and released on the same day. They can be read in either order to complement each other, and inspired a pair of anime films (also released on the same day).