Monogatari Series: Second Season

Synopsis

It's the last day of summer vacation and Koyomi looks for Mayoi to return the backpack she left behind at his house. On the way, he has an encounter with Yotsugi who reveals that just like him and Mayoi, she once died, but while Koyomi was transformed into an immortal being and Mayoi became a ghost, she was revived by Yuzuru as a shikigami to serve under her. With no sign of Mayoi, and with Yotsugi's words on his mind, Koyomi returns home and keeps thinking about what he could do for Mayoi until he is reminded by Shinobu that he has only a few hours to finish his summer homework. He then ponders if he could return to the past to finish it in time and much to his surprise, Shinobu reveals that it is possible. At an abandoned temple, Shinobu opens a portal to the past and they pass by it intending to reach the previous day, but some time later, they realize that they had traveled to eleven years in the past instead. (Source: Wikipedia)