Kayababashi Bridge is the October entry in a twelve-month list of Nihonbashi-area bridges, appearing both on a calendar associated with Mutsuo Koshikawa and on a handwritten note found in an apartment once occupied by Kaga’s mother. Its significance is less geographic than evidentiary: the matching lists connect an object investigated in a murder inquiry with a private clue in Kaga’s family history.

Place in the bridge list

The calendar’s annotations assign a bridge to every month. Kayababashi Bridge appears in October, after Yoroibashi Bridge in September and before Minatobashi Bridge in November. The other entries include Yanagibashi Bridge for January, Asakusabashi Bridge for February, Tokiwabashi Bridge for April, Ichikokubashi Bridge for May, Nishigashibashi Bridge for June, Nihonbashi Bridge for July, Edobashi Bridge for August, and Toyomibashi Bridge for December. Investigators establish that the set consists of bridges in the Nihonbashi area, but initially cannot determine why Koshikawa had them recorded.

The matching note

A separate sheet found in Kaga’s mother’s apartment carries the same month-by-month sequence, including Kayababashi Bridge in October. The handwriting is not Kaga’s mother’s, and Kaga cannot establish whether the note belonged to her. When he shows the list to his cousin Shuhei Matsumiya, the match with Koshikawa’s calendar astonishes them both. Kaga asks Matsumiya to arrange a handwriting analysis, turning the coincidence of two matching lists into a line of inquiry.

Connection to Kaga’s family history

Kaga tells his superiors that his mother had a long-term relationship with Shunichi Watabe, and suspects Watabe may have written the note. At that point, Kaga knows little about Watabe and has been unable to interpret the list. Later, the investigation’s reasoning identifies Watabe as likely to have been Hiromi Asai’s father, Tadao Asai, who was believed to have died by suicide but had in fact used another person’s death to disappear. The bridge list therefore sits at the intersection of two investigations: the meaning of Koshikawa’s calendar and the concealed history of Kaga’s mother’s acquaintance.

A route through the waterways

The bridges also belong to a navigable sequence. During a boat trip, the party passes under Edobashi, Yoroibashi, Kayababashi, Minatobashi, and Toyomibashi before reaching the Sumida River and turning back into the Kanda River. Kayababashi is thus both a named clue on paper and one point in a physical route through the same cluster of waterways. Its place in the repeated list gives the bridge a narrative role as a small, exact connection between objects, locations, and people whose relationships were not initially understood.