The novels, which can be read in either order, are set in parallel worlds
that are created as a result of Koyomi's decision from choosing between his mother Mayumi Takasaki and father Shodai Hidaka following their divorce.

To Every You I've Loved Before
In a world where people regularly travel between slightly different parallel universes, Takasaki Koyomi lives with his mother after his parents’ recent divorce. Awkward and studious, he has trouble making friends at his new school until his classmate Takigawa Kazune seeks him out. She claims that she’s moved here from another world where she and Takasaki are in love… but could he be the one who’s actually from a parallel world instead?

TO Me, The One Who Loved You
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?

(text from Seaven Seas)