The final volume of the Final Season begins when the mysterious underclassman Ougi Oshino draws Koyomi Araragi into a classroom that should not exist at Naoetsu Private High School. This impossible room, a remnant of Koyomi’s past, becomes the stage for Ougi Formula, where the two investigate a bizarre locked-room mystery from two years prior. The investigation forces Koyomi to confront a forgotten cheating scandal that alienated his entire first-year class, revealing the origins of his isolated high school life.

The narrative then shifts to Sodachi Riddle and Sodachi Lost, arcs that introduce the troubled prodigy Sodachi Oikura. Returning to school after a long absence, Sodachi harbors a venomous hatred for Koyomi, though he cannot remember why. With help from Ougi and the sharp-eyed班长 Tsubasa Hanekawa, Koyomi uncovers a devastating forgotten past: in middle school, Sodachi was a childhood acquaintance who lived in an abusive home and used mathematics lessons as a silent cry for help that the oblivious Koyomi failed to answer. The resolution of her tragedy forces Koyomi to accept the limits of his ability to save others from real-world suffering.

In Shinobu Mail, the focus returns to the supernatural. Koyomi’s vampire companion, Shinobu Oshino, vanishes from his shadow. Seeking answers from the enigmatic specialist Izuko Gaen, Koyomi learns of the imminent resurrection of the First Servant, the original vampire oddity killer and Shinobu’s first minion. This ancient being, now a living contradiction of ash and rage, seeks to reclaim his master and destroy Koyomi. The arc culminates in a desperate battle that resolves the four-hundred-year-old grudge between Shinobu and her progeny.

The concluding volume, End Tale proper, is divided into three parts. In Mayoi Hell, Koyomi is murdered and finds himself in a Buddhist inferno, where he reunites with the ghost-turned-deity Mayoi Hachikuji. She guides him through the afterlife to retrieve a necessary tool for his return. Hitagi Date provides a quiet, poignant interlude as Koyomi spends a rare day of genuine, monster-free romance with his girlfriend, Hitagi Senjougahara, before his final confrontation. Finally, Ougi Dark reveals the truth: Ougi Oshino is not the niece of the expert Meme Oshino, but an oddity born from Koyomi’s own repressed self-criticism, a dark reflection created to judge his own hypocritical nature. As the entity known as the Darkness arrives to erase this illegitimate anomaly, Koyomi must accept his flaws and embrace Ougi as part of himself to save her.

The narrative concludes with Zoku Owarimonogatari, an epilogue set after graduation. On the morning of his university entrance exam results, Koyomi falls through a mirror and wakes up in a strange reflection of his town. In this world, everyone he knows is a twisted "opposite": the immature Shinobu is a wise adult, the stoic Yotsugi Ononoki is highly emotional, and his perpetually cheery sister Tsukihi is identical because she has no hidden side. Most poignantly, he meets a cheerful, well-adjusted version of Sodachi Oikura who was adopted by his family. Guided by the reflection of Ougi, Koyomi realizes he did not enter the mirror; he pulled the mirror world out. He must choose to abandon these comforting illusions to return to his imperfect reality, accepting that he cannot save everyone from their regrets.