Ougi Oshino, a mysterious individual claiming relation to Meme Oshino, has inserted themselves into Koyomi’s life and begins to take great interest in both his past and future. An interest which puts him on a collision course with Izuko Gaen.
This portion of the Monogatari franchise is specifically tied to Monogatari: Second Season, Tsukimonogatari, and Koyomimonogatari: The first three stories take place after Koyomi episodes 6 & 7 and focus on Koyomi’s past, the fourth is a recollection of his side of Nekomonogatari (Shiro) told following Tsuki‘s events, and the remaining three continue from Koyomi‘s last episode to conclude the highschool chapter of his life.
More Information:
aniDB (1), (2)
Crunchyroll
Wikipedia