The Kanda River is both a physical route through central Tokyo and a working part of Kyoichiro Kaga’s investigation: its bridges connect a puzzling list of dates and places to the concealed identity of Shunichi Watabe. A boat trip along the river and into the Nihonbashi River makes the geography of the clue tangible, while the case eventually reframes Watabe as the missing father, Tadao Asai.

Setting and Investigative Route

The bridge list Kaga found among his mother’s belongings assigned a different Tokyo bridge to each month. Its locations matched those marked on Mutsuo Koshikawa’s calendar, and Kaga asked the task force to arrange handwriting analysis. The bridges named in the list lay on either the Kanda River or the Nihonbashi River, turning the waterways into a route through the evidence rather than mere background.

With the investigation stalled, Kaga described plans to make another circuit of the bridges by boat. The boat was to depart from Asakusabashi Bridge, placing the detectives at the Kanda River’s edge and giving them a way to follow the sequence in person. As the engine started, the boat slipped past a moored Japanese-style pleasure boat and headed upstream along the Kanda River.

The River Journey

During the ride, Kaga drew Matsumiya’s attention to the contrasting buildings on the riverbanks, some with many windows and others with very few. The exchange shows Kaga using the journey to observe the waterside environment, though the available scene does not establish a definitive explanation for the architectural difference.

After the boat passed Suidobashi Bridge, it continued to a fork where the left-hand channel turned almost at a right angle. That branch led into the Nihonbashi River. The turn gives the investigation’s paper trail a visible geography: the bridge names are not an abstract list but points distributed along connected waterways that the detectives can travel between.

From Geography to Identity

The river route belongs to a broader effort to identify Watabe, a man connected to both Hiromi Asai and Michiko Oshitani. Investigators had initially concentrated on Seizo Naemura, but Yasuyo Miyamoto’s testimony showed that Naemura was not the missing man they sought. Kaga then reconsidered the relationship evidence: Watabe had been connected to Hiromi for at least ten years, appeared to have few other ties, could not be located, and had known Oshitani.

Kaga reasoned that Hiromi’s sustained financial support of Watabe suggested an unusually close bond, such as a lover or a family member. Taking Watabe’s estimated age and his connection to Oshitani into account, Kaga concluded that the simplest explanation was that Watabe was Hiromi’s father, Tadao Asai. The supposed record of Tadao’s suicide came only from the files of Biwa Gakuen Orphanage, without official corroboration; former classmates did not remember that her father had died by suicide. Kaga inferred that another person had died in Tadao’s place and that Tadao had used that death to erase his own identity. The Kanda River’s bridge circuit thus sits within an investigation that moves from matching locations to recovering a hidden family relationship and a falsified death.