"Mouse" is a psychological thriller manga that explores themes of justice, morality, and the nature of evil. The story centers on a world where a gene associated with psychopathy, known as the "Murder Gene," has been identified. Individuals carrying this gene are predisposed to violent and antisocial behavior, leading to widespread fear and discrimination. The narrative follows two primary characters: Michio Yuki, a high school student, and Keigo Torao, a detective.

Michio Yuki is a seemingly ordinary teenager who discovers he carries the Murder Gene. Despite this, he aspires to live a normal life and rejects the violent tendencies associated with the gene. However, his life takes a dark turn when he encounters a serial killer known as the "Escargot," who targets individuals with the Murder Gene. This encounter forces Michio to confront his own potential for violence and the societal stigma surrounding the gene.

Keigo Torao, on the other hand, is a detective determined to eradicate the threat posed by individuals with the Murder Gene. His methods are ruthless, and he operates under the belief that eliminating those with the gene is the only way to ensure public safety. Torao's relentless pursuit of justice often blurs the line between right and wrong, raising questions about the morality of his actions.

As the story progresses, Michio and Torao's paths intersect, leading to a complex dynamic between the two. Michio struggles to maintain his humanity while being hunted by Torao, who views him as a dangerous threat. The narrative delves into the psychological and ethical dilemmas faced by both characters, examining the impact of genetic determinism on identity and behavior.

The plot is driven by a series of violent encounters, moral quandaries, and unexpected twists. The manga explores the consequences of living in a society that fears and ostracizes individuals based on their genetic makeup. It also raises questions about free will, the nature of evil, and the possibility of redemption. The tension between Michio's desire for a peaceful life and Torao's uncompromising pursuit of justice forms the core of the story, creating a gripping and thought-provoking narrative.