While struggling to make friends after his parents’ divorce, Koyomi Takasaki meets Kazune Takigawa, but apparently, they already have. Kazune reveals that she’s from World Line 85, an alternate universe in which she and Koyomi are lovers. But in a reality where moving between dimensions is natural, could Koyomi be the one from another world?
The worlds with the option you 
didn't choose are alternate realities.
Die Welten, für die sie sich nicht
entschieden haben, sind Parallelwelten.
Los mundos que no eliges
en esas elecciones son mundos paralelos.
Los mundos que no eliges
en esas elecciones son mundos paralelos.
Les « mondes parallèles »
Fra tutte queste scelte,
O mundo que dá escolhas
diferentes é o mundo paralelo.
Так появляются параллельные миры —