First Love is Now First Love Letter

Kazuya Kawaguchi has always had a crush on Miki Toda since elementary school, but every time he tries to give her a love letter to confess his feelings, he ends up turning away in fear. Several years later, the two meet at an alumni homecoming. Much to his dismay, Kazuya discovers that Miki doesn't remember who he is, and so puts up an act to hide the fact that the revelation has affected him so.
Meanwhile, Miki garners a vague recollection of Kazuya, and so begins feeling similarly dismayed at the fact that he seemingly doesn't remember who she is, either. But just when Miki thinks the night will end in sheer frustration, Kazuya builds the courage to ask her out on a date. And so begins the awkwardly cute love story of these two incidental lovers, where everything started with a stack of undelivered love letters well over a decade overdue.