Kanashi Kanashi ka

Middle schooler Ai has felt someone's presence ever since he was a child. When he wanders into a parallel universe, he meets a girl named Suzuro who looks a lot like his mother. When Ai finds out that Suzuro is the true daughter of his parents, and that he is in fact from the parallel universe, and was switched with Suzuro in the womb—what will Ai do!?

The story of a boy and girl bound by a quirk of fate.

(Source: Shinshokan)