Minatobashi Bridge is one of twelve bridges in the Nihonbashi area whose names appear month by month on Mutsuo Koshikawa’s calendar and on a matching note Kyoichiro Kaga found in his mother’s apartment. Its significance lies not in an isolated incident at the bridge, but in its position within a repeated sequence that connects Koshikawa, Kaga’s family history, and the investigation’s search for the meaning of the list.

Place in the Twelve-Bridge Sequence

The calendar’s annotations assign one bridge to each month. Minatobashi Bridge is the November entry, between Kayababashi Bridge in October and Toyomibashi Bridge in December. The detectives learn that the named bridges are all in the Nihonbashi area, but their initial inquiry finds no evidence for the search team’s suggestion that the annotations recorded events Koshikawa had attended. The list therefore identifies a geographic pattern without immediately explaining why the bridges matter.

The Matching Note

Kaga has found a sheet listing the same twelve month-and-bridge pairings among the effects in his mother’s apartment. He is careful to distinguish where he found it from who owned it: its handwriting is not his mother’s, and the note’s authorship is uncertain. The perfect correspondence with Koshikawa’s calendar makes the list a link between two otherwise separate sets of evidence. Kaga suspects that Shunichi Watabe wrote it, in connection with his long-term relationship with Kaga’s mother, but admits that he has not worked out what the annotations mean. He asks the task force to arrange handwriting analysis, turning the matching lists into a formal investigative lead.

Route Through the Bridges

The bridge sequence also acquires a physical dimension when the detectives travel by boat. The route passes beneath Edobashi, Yoroibashi, Kayababashi, Minatobashi, and Toyomibashi before reaching the Sumida River, then turns upstream toward the mouth of the Kanda River. Instead of continuing along the usual tourist route toward Tokyo Skytree, the boat turns back into the Kanda River and proceeds beneath Tanagibashi Bridge, returning to Asakusabashi Bridge, where the journey began. Minatobashi Bridge thus appears as one point in a navigable chain, helping place the names on the paper within the city’s connected waterways.

Investigative Significance

The repeated list matters because it carries information across relationships and time without disclosing its purpose. Kaga’s explanation to his superiors later identifies Watabe as someone connected to Hiromi Asai for at least a decade, possibly supported financially by her, and known to Michiko Oshitani. Considering Watabe’s estimated age and disappearance, Kaga concludes that he may have been Hiromi’s father, Tadao Asai, who was believed to have died by suicide. The evidence does not establish Minatobashi Bridge as the site of a specific event; its importance is as one precise entry in a list whose matching versions help connect the investigation’s personal and geographic clues.