"Kannazuki no Shimai - Mad Yakuzaishi Sae" is a manga series that blends elements of supernatural drama, action, and family dynamics. The story centers on Sae Kannazuki, a young woman who works as a yakuza doctor, providing medical care to members of the underworld. Her life takes a dramatic turn when she encounters her estranged younger sister, Yuki, who possesses a mysterious and dangerous power tied to a cursed lineage. The sisters' reunion is fraught with tension, as their shared past is marked by tragedy and betrayal.

Sae and Yuki are descendants of the Kannazuki family, a lineage cursed with the ability to summon destructive entities known as "Mad Gods." These entities are unleashed through the emotional turmoil of the Kannazuki heirs, causing chaos and devastation. Yuki, who has been raised in isolation due to her uncontrollable powers, seeks Sae's help to understand and control her abilities. However, their bond is tested by the resurgence of long-buried family secrets and the interference of rival factions within the yakuza world.

As the story progresses, Sae and Yuki navigate a dangerous landscape of supernatural threats and human adversaries. Sae's role as a yakuza doctor places her in a unique position to mediate between the criminal underworld and the supernatural forces at play. Her medical expertise and moral compass often clash with the violent realities of her environment, creating a complex dynamic between her professional duties and personal struggles. Meanwhile, Yuki's journey of self-discovery is marked by internal conflict, as she grapples with the destructive potential of her powers and the weight of her family's legacy.

The narrative explores themes of redemption, familial loyalty, and the consequences of wielding power. Supporting characters, including yakuza members, supernatural entities, and other individuals tied to the Kannazuki curse, add depth to the story by providing contrasting perspectives on morality and survival. The interplay between the sisters' relationship and the external forces threatening their lives drives the plot forward, culminating in a series of high-stakes confrontations that challenge their resolve and reshape their understanding of their shared destiny.