Every action has a consequence. It's August 20th and the last day of the summer holidays for Koyomi Araragi.
After the realization that he hasn't started any of his holiday assignments, with the first day of school tomorrow looming over him, he decides to cheat the system by calling the help of Shinobu to travel back in time.

Except it seems they've travelled back too far, so far back that it's 11 years in the past and the date is May 13th.
By sheer luck the date is the day before Mother's Day when Mayoi was killed in a car accident. Deciding to alter the past, Koyomi must face the effects it has on the future.