"The Detective Is Already Dead" centers around Kimihiko Kimizuka, a high school student who inadvertently becomes entangled in a world of mysteries and supernatural phenomena. Three years prior to the main events, Kimihiko was recruited as an assistant to Siesta, a brilliant and enigmatic detective who traveled the globe solving extraordinary cases. Together, they faced bizarre incidents involving criminal organizations, supernatural entities, and advanced technology. However, their partnership came to an abrupt end when Siesta died during a mission, leaving Kimihiko to return to a seemingly ordinary life.

The story resumes with Kimihiko attempting to move on from his past, but his life takes a dramatic turn when he encounters Nagisa Natsunagi, a girl who bears a striking resemblance to Siesta. Nagisa claims to possess Siesta’s heart, which was transplanted into her after the detective’s death. This revelation reignites Kimihiko’s connection to the world of mysteries, as Nagisa seeks his help to uncover the truth behind her condition and the lingering shadow of Siesta’s legacy. As they delve deeper, they encounter a secretive organization known as SPES, which is linked to many of the cases Siesta once investigated. SPES seeks to manipulate humanity through advanced technology and supernatural means, posing a significant threat to the world.

Throughout the series, Kimihiko and Nagisa are joined by other key characters, including Charlotte Arisaka Anderson, a skilled hacker and former ally of Siesta, and Yui Saikawa, a classmate of Kimihiko’s who becomes involved in their investigations. The narrative explores themes of loss, identity, and the enduring impact of past relationships, as Kimihiko grapples with his memories of Siesta and his evolving bond with Nagisa. The story balances episodic mysteries with an overarching plot, gradually revealing the true nature of SPES and its connection to Siesta’s death. The series combines elements of detective fiction, science fiction, and emotional drama, creating a multifaceted narrative that keeps readers engaged.