After the sacrificial death of his mentor Tsukikage, Rekka of the Crimson Lotus anti-demon brigade struggles with guilt while investigating a series of disappearances in a remote village. Along the way, he discovers that not all demons are monsters, forming an unlikely bond with a peaceful creature. However, this newfound understanding is challenged when the Black Steel, a ruthless assassin brigade, arrives to claim the demon as their prey.
The Crimson Lotus attempts to recruit the skilled demon hunter Kureha, who initially refuses to join due to her disdain for male leadership. The group soon embarks on a mission to defeat Burai, a powerful demon terrorizing Azuchi by kidnapping women. After surviving a treacherous trap and a demonic possession, Rekka and Kureha must put aside their differences to confront Burai, leading to a climactic battle where Rekka's shadow techniques prove decisive.
Rekka and the Crimson Lotus uncover the Black Steel's sinister plot to create a superhuman race by fusing humans with demons, a process that releases defective monsters into the world. Driven by a desire for justice and Haruka's need for vengeance against the leader Yasha, the team launches a decisive attack. The battle culminates in the destruction of the Black Steel, ending their cruel experiments and avenging the lives they destroyed.