Mononoke is a spin-off of the Bakeneko story arc of Ayakashi - Japanese Classic Horror. It follows the Medicine Seller as he deals with various spirits or "Mononoke" in feudal Japan. Just like in the Bakeneko story, he combats with a series of tools and his vast knowledge of the supernatural on his side, but can only slay the Mononoke when he uncovers its form (Katachi), the truth behind its appearance (Makoto) and the reason for its unusual behavior (Kotowari). In other words : Truth, Form and Regret.

[Source: Wikipedia]

The Ōoku —

A harem swirling with intense emotions.

The Ōoku is where beautiful and talented women are gathered from all across the land to bear the heir of His Majesty, the ruler. At the same time, it is an important place that plays a crucial bureaucratic function. Stepping into this unusual society with its own rules are young newcomers Asa (Tomoyo Kurosawa) and Kame (Aoi Yuki). Blessed with both intelligence and beauty, Asa aims to improve her standing, while Kame searches for a sense of belonging in this place she has always dreamed of. On the day of their induction, these two girls of opposite personalities are forced to participate in a ceremony to become part of the group by sacrificing their most cherished belongings to the Water Goddess, a deity worshiped within the Ōoku. As a result of this event, a bond forms between the two girls.

The elder Utayama (Mami Koyama) oversees the women while prioritizing the prosperity and continuation of the Ōoku, but she conceals something behind her stony facade. Slowly, over time, something uncanny begins to envelop the women. As their powerful emotions accumulate each night, the strange sound of a spinning paper umbrella echoes from the shadows and the maidservants begin to lose their minds, as though they are possessed.

Eventually, a pivotal tragedy occurs. The Medicine Seller (Hiroshi Kamiya) proceeds to the heart of the Ōoku, but he cannot draw the Sacred Sword to exorcise the mononoke unless he learns the being’s “Form,” “Truth” and “Reason.” When the Medicine Seller uncovers the frightening and heartbreaking truth hidden in the Ōoku, the ceremony of exorcism and redemption begins.

[Source: Netflix]

Shortly after the intense showdown in Mononoke the Movie: The Phantom in the Rain, the Medicine Seller appears once again in the inner chambers of the Ōoku. Within, dynamics are shifting in the wake of the previous incident. Otomo Botan, a woman from a distinguished family, succeeds Utayama as the new director and begins enforcing a strict leadership style rooted in discipline and balance. This creates growing tension between her and Fuki, a seasoned courtesan who once enjoyed the exclusive favor of the emperor, causing a rift that deepens with each passing day. As the selection process begins for a guardian to look after the newborn child of the emperor’s wife, Empress Yukiko, an unexpected crisis suddenly turns Fuki’s situation upside down. Councilor Otomo views her delivery of an "unwanted child" as a threat, and Fuki becomes the target of a web of schemes spun by the men in power.

Conflicting motives and desperate attempts to “put out the fire” quickly spiral out of control. Meanwhile, a series of inexplicable events occurs, with people spontaneously combusting and being reduced to ashes. The Medicine Seller suspects the work of a Mononoke and sets out to contain it, but he is obstruced by elusive apparitions that seems to act as a group. In reality, this mysterious phenomenon is the children of the Hinezumi, the fire mouse. They seem to be searching for their mother, but she does not reveal herself easily. Why does Hinezumi target those who would harm newborns? What tragedy drives her relentless sorrow as she sets herself ablaze? The Medicine Seller delves into the darkness lurking within the Ōoku to solve the mystery and uncover the mononoke's Form, Truth and Reason: the three elements required to cut it down for good.

[Source: Netflix]