"Mouse" is a psychological thriller manga that explores themes of morality, justice, and the nature of evil through a gripping and dark narrative. The story centers on a world where a rare genetic mutation, known as the "Murder Gene," exists in a small percentage of the population. Individuals carrying this gene are predisposed to violent and psychopathic tendencies, making them potential threats to society. The discovery of this gene leads to widespread fear and discrimination, as well as the emergence of vigilante justice targeting those suspected of carrying it.

The protagonist, Michio Yuki, is a high school student who becomes entangled in this dangerous world after a tragic encounter with a serial killer. Yuki's life takes a drastic turn when he discovers that he possesses the Murder Gene, a revelation that forces him to confront his own potential for violence. Determined to use his abilities for good, Yuki embarks on a mission to hunt down other carriers of the gene who have succumbed to their darker impulses. His journey is fraught with moral dilemmas, as he struggles to balance his desire for justice with the fear of becoming the very thing he seeks to destroy.

Throughout the series, Yuki encounters a range of characters who challenge his beliefs and motivations. Among them is Kaito Minagawa, a detective who shares Yuki's goal of eliminating dangerous carriers but employs ruthless methods that blur the line between justice and vengeance. Another key figure is Sanae Miyoshi, a young woman with a tragic past who becomes both an ally and a source of emotional conflict for Yuki. The interactions between these characters highlight the complexities of human nature and the fine line between heroism and villainy.

The plot is driven by a series of intense confrontations and investigations, as Yuki and his allies track down individuals who have used the Murder Gene to commit heinous crimes. Each case delves into the psychological and societal implications of the gene, raising questions about free will, responsibility, and the ethics of genetic determinism. The narrative is punctuated by moments of high tension and graphic violence, reflecting the grim and unforgiving world in which the characters operate.

As the story progresses, Yuki's mission becomes increasingly personal, culminating in a climactic showdown with a powerful and enigmatic antagonist who embodies the darkest aspects of the Murder Gene. This confrontation forces Yuki to confront his own limits and the true nature of his quest, leading to a resolution that is both thought-provoking and emotionally charged. "Mouse" is a compelling exploration of the human condition, offering a stark and unflinching look at the consequences of genetic predisposition and the struggle for redemption in a world defined by fear and violence.