"Hero Mask" is a crime thriller anime series set in a fictional version of London, blending elements of mystery, action, and political intrigue. The narrative revolves around a series of murders connected to a mysterious substance called "Bio-Mask," which grants its users superhuman abilities. The story begins with the murder of a prominent politician, leading Detective James Blood of the Metropolitan Police to investigate the case. As he delves deeper, he uncovers a conspiracy involving high-ranking officials, secret organizations, and the enigmatic Bio-Mask technology.

James Blood is joined by Sarah Sinclair, a prosecutor with a personal connection to the case, as they work to unravel the truth behind the murders. Their investigation reveals that the Bio-Mask is part of a larger experiment tied to a shadowy group known as "Section 9," which seeks to manipulate political power and societal control. The series explores themes of corruption, identity, and the ethical implications of advanced technology, as the protagonists face off against powerful adversaries who exploit the Bio-Mask for their own gain.

The plot is driven by a series of twists and revelations, with characters frequently questioning their loyalties and motivations. Key figures include Leonard Truman, a former intelligence officer with a hidden agenda, and Monica Campbell, a scientist whose research on the Bio-Mask plays a pivotal role in the story. The narrative builds toward a climactic confrontation between the protagonists and the forces behind the conspiracy, culminating in a resolution that ties together the various threads of the mystery while leaving room for further exploration of the world and its conflicts. The series maintains a fast-paced, suspenseful tone, with a focus on intricate plotting and character-driven drama.