On August 20th, on his way to delivering Mayoi what she had forgotten at his home, Koyomi encounters Yotsugi Ononoki, a familiar girl. Koyomi spends the day buying Yotsugi ice cream instead of doing what he originally planned to do. Unable to find Mayoi, Koyomi gives up and heads home and realizes that he has not done any of his summer assignments, even though tomorrow was the first day of school. With help from Shinobu, Koyomi travels back to the past so that he can finish them, but instead, they end up traveling back 11 years ago!
The date is May 13th, the day before Mother's Day when Mayoi was killed in a car accident on her way to see her mother. Koyomi realizes that he can save Mayoi by changing the past, but what will be the outcome of his actions...