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?
I realized that the reason I could 
take joy in a stranger's happiness
Ich kann mich über das Glück
von jemand anderem freuen,
Ser capaz de algo así
Ser capaz de algo así
C’est parce que je nage moi-même
dans le bonheur
Mi sono accorto che sono contento
 della felicità di uno sconosciuto
E por ficar feliz pela felicidade dos outros,
А ещё я понял, что обрадовался,