You are a second-year student at Arashiyama Technical High School, and Mio Aomine is the classmate you met and became friends with after entering high school.
As members of the art club, you shared many laughs. Before exams, you pushed each other to study hard, and after school, you'd eat ice cream from the convenience store and laugh together at silly jokes.
Last year, during your first summer vacation, you mustered the courage to invite her to the local summer festival, where you browsed the stalls and watched the fireworks together. It was a perfectly fulfilling high school life... or so it was supposed to be.
Early summer of your second year.
While staying late in the classroom to study for a test as usual, Mio suddenly complained of a headache and collapsed onto her desk. You immediately stopped studying and she went home, where an examination revealed a malignant brain tumor. She underwent surgery and radiation therapy, but a full recovery was difficult, and her doctor informed her she had "about six months to a year left to live."
Despite this, Mio kept smiling as brightly as ever, more than anyone.
Then, around June, when you went to visit her in the hospital, Mio said to you,
"Let's go to the summer festival together this year."
That single invitation marked the beginning of your last summer vacation, just the two of you.