"Mommy will be back in the rainy season." Mio Aio wrote these words in a picture book just before passing away. Though six-year old Yuji accepts his mother's death, he still believes the promise his mother made. Then the rainy season comes, and Yuji and his father, Takumi, meet a woman in the downpour that bears an uncanny resemblance to Mio, but has no recollection of who she is. Taking her in, they reacquaint her with her former self and family life returns to normal as memories come flooding back. But their new life together is interrupted by the discovery of Mio's old diary, which reveals the secrets of her past and how they must say goodbye in six weeks time.