Two months after the internal power struggle within the criminal organization Ryuugen, narcotics investigator Kai has been discharged from the hospital and cleared to return to work at the Kanto Shinetsu Narcotics Control Division. However, he has lost all memory of his partnership with Haru Kurabayashi, known as Hal, and the events of the previous year. Meanwhile, Hal has submitted his resignation, which his superior, Chief Hickey, has refused to process, instead placing him on paid leave without his knowledge. Hal has gone underground, disguising himself as a drug dealer to conduct an independent investigation into a potent new synthetic drug appearing on the streets, marked only with the initials DF, which stands for Dragon Fruit.

While working an undercover case on the DF narcotic, Kai encounters Hal in his guise as a dealer. Despite having no recollection of him, Kai feels a strange and powerful sense of familiarity. The DF drug case deepens when Hal is framed for murder. With the assistance of Chief Hickey, Hal clears his name, but as a condition of his exoneration, he is forced to return to active duty, once again partnered with the amnesiac Kai. The narrative arc follows the pair as they work to dismantle the distribution network of DF, a crude and dangerous drug being sold under the tarnished reputation of the fallen Ryuugen organization. The investigation brings them into conflict with the Toukoku-kai, a new syndicate exploiting the DF trade, and forces them to confront the lingering shadows of Ryuugen.

A notable subplot involves investigator Konosuke Nagakura, a transfer from the Kinki region. Once an ace detective, Nagakura was transferred after brutally assaulting the gang members who kidnapped his college-aged brother. He found his brother three months later, but the young man had been driven insane by sustained drug use and assault, leaving him hospitalized and unable to recognize his family. This trauma drives Nagakura to maintain a controversial reliance on an underworld informant, the same person who provided the crucial tip that saved his brother, even though he does not know the informant's true identity. The hunt for the source of Dragon Fruit ultimately reveals that the drug is a shoddy imitation meant to capitalize on the Ryuugen name, and the conspiracy extends to individuals within the police force itself, including those close to the deceased Ryuugen leadership. The story resolves the immediate threat of the DF drug ring and sees Hal officially reinstated, though Kai's lost memories of their past together remain a fragile and unresolved element between them.