Takasu Goro, once forced out of boxing by a fixed match, now runs a front as a private detective while blackmailing targets ranging from students to yakuza bosses. Feared for his ruthless methods, he’s known as “the dirty one”. His intellectually disabled sister Keiko befriends a man named Osho at her favorite riverside spot. When a yakuza executive is killed, Goro tries to extort the boss for 100 million yen—triggering Keiko’s kidnapping.