"Collar x Malice" is a mystery thriller anime series set in Shinjuku, Tokyo, where a series of violent crimes known as the "X-Day Incidents" have plunged the city into chaos. The story centers around Ichika Hoshino, a young police officer who becomes entangled in the investigation after being attacked and fitted with a poisonous collar by an unknown assailant. The collar serves as both a threat and a clue, forcing her to collaborate with a group of former detectives who have been unofficially investigating the X-Day Incidents.

The group, known as the "Adonis," consists of five men: Aiji Yanagi, a calm and composed former detective; Takeru Sasazuka, a sharp-tongued and tech-savvy investigator; Kei Okazaki, a flirtatious and resourceful ex-cop; Mineo Enomoto, a cheerful and impulsive former officer; and Kageyuki Shiraishi, a mysterious and enigmatic figure with ties to the incidents. Each member brings unique skills and perspectives to the investigation, and their interactions with Ichika reveal their personal motivations and connections to the case.

As the investigation unfolds, Ichika and the team uncover a complex web of conspiracies, political corruption, and psychological manipulation tied to the X-Day Incidents. The crimes are linked to a shadowy organization that seeks to expose societal flaws and provoke change through violence. The narrative delves into themes of justice, morality, and the consequences of systemic failure, as the characters grapple with their own beliefs and the ethical dilemmas posed by the case.

The plot is driven by a combination of detective work, interpersonal dynamics, and high-stakes confrontations. Ichika's role as both a victim and an investigator adds tension, as she must navigate her own vulnerability while contributing to the team's efforts. The series balances suspense with character development, exploring the backgrounds and motivations of each member of Adonis and their relationships with Ichika. The resolution of the X-Day Incidents hinges on uncovering the truth behind the organization's motives and dismantling its influence, leading to a climactic confrontation that tests the limits of the characters' resolve and principles.