The Detective Boys visit a chess tournament in the Nishi-Topia building together with their teacher Kobayashi to support Inspector Shiratori, who wears a knight keychain as a good-luck charm – a gift from Kobayashi. The tournament participants, including Shiroi with his rook keychain, Ogawara with a distinctive king glass, and Kishitani, who believes in knightly superstitions, stand out due to their quirks. During the game, the song „Amazing Grace“ suddenly plays, reminding everyone of former participant Saso Ryoichiro, who lost his life after a bitter defeat. Shortly afterward, Kishitani Naoto is found dead in the men's restroom on the 17th floor: killed by crossbow arrows to the chest and legs. Next to the body lie the murder weapon, a chess magazine, and Shiratori’s lost knight keychain. Conan and MPD Superintendent Kuroda immediately notice inconsistencies – such as a „Cleaning“ sign that differs from those in other floors, and bloodstains suggesting the victim left a clue: in the chess magazine, Kishitani left a bloody handprint with a magatama pattern between his fingers.

The investigation focuses on four suspects: Shiratori, whose keychain was found at the crime scene and who was alone during the time of the murder; Kisaki, whose jacket has a tear that could have been caused by a crossbow bolt; Ogawara, who seemingly fell victim to a crossbow attack during the tournament – possibly a staged act; and Shiroi, who claims to have been in the women’s restroom without witnesses during the crime. A key piece of evidence is two identical glasses cases in the trash: one full of glass shards, the other empty. Conan combines this detail with the bloody handprint and realizes: the magatama pattern perfectly matches the base of Shiratori’s knight keychain. This makes it clear that Kishitani identified the culprit with this symbol in his final moments.

The episode ends with a cliffhanger: Kuroda makes cryptic hints to Wakasa about an incident 17 years ago, while the Black Organization (RUM) is already planning the elimination of a „suspicious person.“ A preview of the next episode promises the resolution of Kuroda’s 17-year-old connection to the case.