Since the Great Tohoku (East Japan) Earthquake deprived Sonomi of her best friend Kyoko, she has lived without accepting the friend’s death.

One day, a time capsule is found; It contains letters they wrote to each other to be read in the future.

With the discovery, never-fading beautiful memories come back. At the same time, an irreparable mistake that Sonomi cannot regret enough comes back as well.

Agonizing over the past, she learns about the Drifting Post, a mailbox that receives letters containing undelivered messages and pent-up feelings to those deceased in the earthquake. To convey her feelings to Kyoko and move on from the painful past, Sonomi makes up her mind to deliver her letter and she sets out for the Drifting Post.