A long-forgotten race, the Hotaru-Raikō, part human and part firefly, dwell secretly alongside man. This noble clan once lived near the Tree of Life. Now, they make their home in a thicket near the Fukui Castle moat in the Echizen Province, just north of the capital city of Kyōto, Japan. Often mimicking man, they have begun to adopt the budding 'way of the warrior' philosophy.

Though it is a time of peace and the cultural pursuit of beauty, calamity looms deep in the forest. A foul creature escapes the bottomless pit, and a traitor emerges among the Raikō clan. Seduced by a spider yōkai who seeks to release an unholy army from the abyss, the traitor helps her form the treacherous Tsuchigumo clan. With their help, the temptress assassinates the Hotaru-Raikō lord and his faithful samurai. In the wake of this devastation, the Daimyō's 18-year-old daughter is tasked by an angel to defeat the spider yōkai, destroy her traitorous clan, and raze the Abaddon gate to the ground.

Can she rise through the grief to lead the remnants of her father's army as onna-musha? Who will stand by her side? What gift does the angel bring her? Will the Hotaru-Raikō find the gate and complete their quest in time?