Masato Kuroda, a homicide investigator with the Tokyo Metropolitan Police, carries a unique and disturbing ability. As a child, he was locked inside a house with his murdered family for several days, a trauma that left him hypersensitive to the scent of death. This manifests as the ability to see the odor of corpses as red lines, allowing him to locate a dead body faster than anyone else. Seeking to make use of this talent, he is reassigned to a newly created department focused on unusual missing persons cases, the Special Search and Investigations Office, also known as the S&I Office.

There, Kuroda is partnered with Ikaga, a brilliant but eccentric criminal profiler whose analytical mind allows him to reach a perpetrator faster than anyone else. The story follows this mismatched duo as they investigate a series of disappearances and murders where the line between life and death is dangerously blurred. Their cases range from voluntary disappearances that turn into kidnappings to other grim mysteries that force them to confront the darkest aspects of human nature.

As they work together, an undercurrent of tension runs between them. Ikaga, ever perceptive, senses that the stoic Kuroda is hiding a deep secret behind his flawless facade. The central mystery of the narrative is the Suicide Line, a shadowy organization that Kuroda believes holds the key to his past. He secretly attempts to make contact with this group, hoping to find answers about the massacre of his family. His quest leads him into danger, culminating in his kidnapping by an unexpected party. Simultaneously, Ikaga pursues his own independent investigation into the organization, seeking to uncover the truth about both the Suicide Line and the disappearance of his own uncle.

The intertwined investigations ultimately expose the culprit behind the Kuroda family massacre and reveal the full nature of the Suicide Line, forcing both men to confront the painful connections between the living and the dead that have defined their lives. The narrative builds to a conclusion where all mysteries are brought to light, guided by the red lines that only Kuroda can see.