"Owarimonogatari" is the fifth installment in the Monogatari series, adapting the final arcs of Nisio Isin's light novels. The narrative is divided into two main storylines, each exploring the mysteries surrounding Koyomi Araragi and his interactions with the supernatural. The first arc, "Ougi Formula," delves into a long-standing mystery from Araragi's past. During his first year of high school, he encountered a locked classroom that no one else seemed to notice. Years later, Ougi Oshino, a mysterious girl claiming to be the niece of Araragi's mentor, Meme Oshino, brings this incident back to light. Together, they investigate the classroom's significance, uncovering a tragic story tied to a former student named Izuko Gaen, who was involved in a series of events linked to the supernatural.

The second arc, "Sodachi Riddle," shifts focus to Araragi's middle school years, introducing Sodachi Oikura, a former classmate who harbors deep resentment toward him. Through a series of flashbacks, the story reveals how Araragi and Sodachi's paths crossed during a math competition and how their relationship deteriorated due to misunderstandings and personal struggles. Sodachi's family issues and her eventual disappearance are central to this arc, with Araragi attempting to reconcile his guilt and uncover the truth behind her suffering. The narrative intertwines past and present, highlighting the emotional weight of their shared history.

The final arc, "Sodachi Lost," continues to explore Sodachi's story, focusing on her return to Araragi's life and her confrontation with the unresolved trauma of her past. This arc ties together the themes of memory, guilt, and redemption, as Araragi seeks to make amends for his perceived failures. Throughout the series, the interplay between Araragi, Ougi, and Sodachi drives the narrative, with Ougi's enigmatic presence serving as a catalyst for uncovering hidden truths. The story maintains a balance between supernatural elements and deeply personal character drama, emphasizing the consequences of past actions and the complexities of human relationships.