Michiko Oshitani, who lived in Shiga Prefecture, is found dead at an apartment in Tokyo. She was strangled to death. Mutsuo Koshikawa is the owner of the apartment and he goes missing. Detectives including Yuhei Matsumiya who is a cousin of Detective Kyoichiro Kaga , have difficulties in solving the case. They can't find a connection between Michiko Oshitani and Mutsuo Koshikawa. They soon learn that Michiko Oshitani came to Tokyo to visit Hiromi Asai . They graduated from the same school, but there isn't a connection between Hiromi Asai and Mutsuo Koshikawa. Detective Yuhei Matsumiya finds something near Michiko Oshitani's body. That something has the names of 12 bridges surrounding the Nihonbashi area of Tokyo. Detective Kyoichiro Kaga learns of the clue and becomes agitated. There's a connection with his late mother