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 saw your aquamarine ring then, and 
I copied you by getting one of my own.
Den Aquamarin-Ring,
den ich damals gesehen habe,
Decidí imitar el anillo de aguamarina
que te vi entonces.
Decidí imitar el anillo de aguamarina
que te vi entonces.
J’ai acheté
la même bague aigue-marine
Quando l'ho fatto, ho notato
 il tuo anello con l'acquamarina
E após ver seu anel com
a pedra água-marinha,
Тогда мне на глаза
попалось кольцо с аквамарином…