Despite his marriage, Satoru Sakuma, a timid school teacher, often dreams about a high school girl under the blooming cherry blossom trees. One day, Satoru witnesses his drunken wife accompanying another man in Yokohama’s red-light district, prompting him to run away. From Sendai to Osaka, he tours around Japan before eventually returning to his hometown of Odawara, where he meets a girl with an uncanny resemblance to the one in his dreams.

Realizing his love for his wife after almost crossing the line with the mysterious high school girl, Satoru plucks up his courage and returns back to her just in time for the start of a new school year. Just as he settles back to his normal and peaceful life, Satoru once again crosses paths with the high school girl he met in Odawara, Aki Aioi, who is now one of his students. At the same time, a mysterious and deadly disease has been spreading across Japan in the exact order that Satoru had traveled in. As Satoru tries to figure out his connection to this phenomenon, his new student seems to be in the know, but she will not divulge why.