The Sumida River is both a setting for the detectives’ investigation and part of the route that makes a recurring list of bridge names legible. As a waterway linking the named bridges to the Kanda River, it turns an apparently static clue into a traversable sequence; elsewhere, the investigation’s search for Shunichi Watabe follows a separate but decisive connection from Hiromi Asai to her father, Tadao Asai.

Setting and Route

During a boat trip, the detectives pass beneath Edobashi Bridge, Yoroibashi Bridge, Kayababashi Bridge, Minatobashi Bridge, and Toyomibashi Bridge before reaching the Sumida River and heading upstream to the mouth of the Kanda River. Instead of continuing along the usual tourist route toward Tokyo Skytree, the boat turns left into the Kanda River, passes Tanagibashi Bridge, and returns to Asakusabashi Bridge, where the trip began. The journey therefore forms a circuit rather than a simple passage downstream, connecting the bridges as places encountered in sequence.

The boat operator, Fujisawa, identifies another possible excursion: a route from the Sumida River into Onagi River, where a lock at Ogibashi lets visitors watch the water level rise or fall. His suggestion places the river within a broader network of navigable waterways, although the detectives’ trip itself is undertaken as part of their work.

The Bridge-Name Clue

The route echoes a list Kyoichiro Kaga found in his mother’s apartment. The note assigns a bridge to each month, and its named bridges and dates match those on Mutsuo Koshikawa’s calendar. Kaga does not know who wrote the note; its handwriting is not his mother’s. The match makes the list a significant investigative clue, while the boat journey supplies a concrete way to think about how the bridges relate spatially.

The river route does not itself establish the note’s meaning or authorship. Kaga’s explanation to his superiors is that his mother had a long-term relationship with Shunichi Watabe, and he suspects Watabe may have written the list. He asks for handwriting analysis because he knows little about Watabe and has been unable to interpret the note. The distinction between a mapped route and an unresolved document preserves the clue’s uncertainty even as the investigation narrows.

From Watabe to Tadao Asai

The investigation initially concentrates on Seizo Naemura, a missing man believed to have connections with Hiromi Asai and Michiko Oshitani. After Yasuyo Miyamoto’s testimony shows that Naemura is not the person they seek, Kaga reconsiders the relationships more directly. He reasons that Hiromi had been connected to Watabe for at least ten years and may have supported him financially; given Watabe’s estimated age, disappearance, and acquaintance with Michiko, Kaga concludes that the likeliest explanation is a close family relationship.

Kaga identifies Shunichi Watabe as Tadao Asai, Hiromi’s father. Tadao was believed to have died by jumping from a building near his home, but the account of that death appeared only in the files of Biwa Gakuen Orphanage and had no official records to support it. Hiromi’s former classmates also did not remember her father’s suicide. These discrepancies lead Kaga to infer that the death happened elsewhere and that the dead person was someone other than Tadao: Tadao had used another person’s death to erase his own identity and remain hidden. The Sumida River’s bridge circuit belongs to the investigation’s geography; the decisive identification emerges from reconstructing the human connections behind the clues.