The story follows the investigations of Narihisago, a renowned detective now in prison, who is tasked with diving into the id wells of various serial killers. Two years prior to the current events, Narihisago's daughter Muku was brutally murdered by a serial killer, leading Narihisago's wife to commit suicide. These deaths prompted him to hunt down and murder the killer, earning him his prison sentence. He is still depressed and haunted by his wife and daughter's deaths, but also uses this as motivation to take his work seriously and help stop serial killers; however, as a result of the deaths of his family, he bears a grudge against all serial killers, which occasionally manifests as an uncontrollable urge to kill. Despite this, however, it is shown that the majority of Narihisago's colleagues have great respect for his work. Rookie field analyst Hondomachi becomes an important character during the series after having a brush with death in a serial killer case.