The Ootsuji Detective Agency is facing a downturn in business, prompting Soutarou and his young partner, Saki, to accept any case that comes their way, regardless of its apparent risks. Amidst the city's festival atmosphere, a string of mysterious disappearances unfolds unnoticed by the public eye. Tasked by a journalist to delve into the enigmatic underworld known as the Dark, Soutarou delegates the investigation to Saki, only to regret it when Saki himself vanishes.
Now, compelled by duty, Soutarou must navigate the treacherous depths of the Dark, confronting its dangerous inhabitants such as the nefarious Laughing Masks gang and the formidable Demon 463 armored train piloted by renegade police officers.