"Yūkyū no Toki" is a historical fantasy manga set in a richly detailed world that blends elements of Japanese mythology, time travel, and political intrigue. The narrative centers on Toki, a young woman from modern-day Japan who is mysteriously transported back in time to the Heian period. Upon her arrival, she discovers she possesses a unique ability to manipulate time, a power tied to an ancient artifact known as the "Eternal Hourglass." This artifact is sought after by various factions, including the imperial court, powerful clans, and supernatural entities, all of whom believe it holds the key to controlling the flow of time itself.

As Toki navigates this unfamiliar and dangerous era, she becomes entangled in a complex web of alliances and rivalries. Her presence disrupts the balance of power, drawing the attention of key historical figures and mythical beings. Among them is Minamoto no Yoshitsune, a legendary warrior who becomes both her protector and a pivotal figure in her journey. Together, they uncover a prophecy that foretells the collapse of the temporal fabric if the Eternal Hourglass falls into the wrong hands.

The story explores themes of destiny, sacrifice, and the consequences of altering history. Toki's struggle to understand her role in this ancient world is compounded by her growing attachment to its people and her realization that her actions could irrevocably change the future. The narrative is driven by a series of escalating conflicts, including battles against malevolent spirits, political machinations within the imperial court, and the looming threat of a temporal cataclysm.

The manga features a diverse cast of characters, each with their own motivations and connections to the central plot. Toki's development from a bewildered outsider to a determined heroine is central to the story, while supporting characters such as Yoshitsune, the enigmatic courtier Fujiwara no Michinaga, and the spirit guardian Kaguya provide depth and complexity to the narrative. The interplay between historical accuracy and fantastical elements creates a compelling backdrop for the unfolding drama, blending action, romance, and philosophical inquiry into a cohesive and engaging tale.