A few months after he first met Mayoi Hachikuji (Bakemonogatari), Araragi set out to deliver something to the young lady. Unable to find her, Araragi gives up and heads home and realises that he has not done any of his summer assignments even though today is the last day of summer break. With help from Shinobu, Koyomi travels back to the past so that he can finish them, but instead, they end up travelling back a full eleven years! The current date is May 13th. It is the day before that Mother's Day when Mayoi left to go see her mother. Koyomi realises that he has an opportunity to save Mayoi! But is he aware of what may result when you change the past like that?
