"Mouse" is a psychological thriller anime that delves into the dark and complex world of serial killers, genetic manipulation, and moral ambiguity. The narrative centers around a society grappling with the emergence of individuals born with a specific gene, referred to as the "psychopath gene," which predisposes them to violent and antisocial behavior. The story explores the ethical dilemmas surrounding the identification and treatment of these individuals, often blurring the lines between justice and vengeance.

The protagonist, a young man named Yohan, becomes entangled in this grim reality after a traumatic encounter with a notorious serial killer known as "The Headhunter." This event sets off a chain reaction, leading Yohan to confront his own potential for violence and the possibility that he may carry the psychopath gene. As the plot unfolds, Yohan crosses paths with a diverse cast of characters, including law enforcement officers, genetic researchers, and other individuals affected by the gene. Each character brings their own motivations and moral perspectives, contributing to the series' intricate web of alliances, betrayals, and moral quandaries.

The narrative is structured around a series of interconnected arcs, each revealing more about the origins and implications of the psychopath gene. Themes of free will, determinism, and the nature of evil are explored through the characters' struggles and choices. The series also examines the societal impact of labeling individuals as inherently dangerous, raising questions about prejudice, discrimination, and the potential for redemption.

As the story progresses, Yohan's journey becomes increasingly intertwined with the broader conflict between those seeking to eradicate the gene and those advocating for a more nuanced understanding of human behavior. The tension builds towards a climactic confrontation that forces characters to confront their deepest fears and desires, ultimately challenging the audience to consider the complexities of human nature and the consequences of playing god with genetics.