Conan and his friends assist Inspector Shiratori at a chess tournament, where they solve a murder committed during the break with the help of Director Kuroda and Professor Agasa. Afterwards, Kuroda reveals to Conan details about an unsolved double murder 17 years ago in the USA: The billionaire Amanda Hughes and the shogi master Haneda Koji were found murdered in a hotel. Kuroda, who was present at the time, discovers Haneda's body in a deliberately vandalized bathroom. A broken mirror with the inscription "PUT ON MASCARA" points to the missing letters "ASACA RUM." He encounters Amanda's injured bodyguard Asaka, who mistakenly believes him to be the murderer. Kuroda decides to secretly bring Asaka to safety. However, their escape in a taxi is pursued by the mysterious organization of the one-eyed RUM, who suspect a GPS tracker in Asaka's luggage. An accident caused by the organization leads to Kuroda's ten-year coma; Asaka manages to escape during this time.

In the present, Conan analyzes Amanda's cryptic dying message – lipstick marks on her watch and a chess piece. He interprets the specific shape of the imprint on the watch face as a clue to cut off the ends of the word "MOUTH," which, combined with "WATCH," results in "WATCH OUT" (Warning/Danger). Kuroda confirms this and links the warning to a man with strikingly "snake-like" eyes whom he briefly encountered. He also expresses strong suspicion that the teacher Rumi Wakasa might be the missing bodyguard Asaka, supported by her conspicuous behavior. A sudden call from Professor Agasa reveals that Wakasa has unexpectedly disappeared, damaging Kobayashi's umbrella in the process but leaving her own striped umbrella behind.

This striped umbrella becomes the target of the organization: RUM, supported by the snipers Chianti and Korn, orders the killing of the presumed Asaka (Wakasa). They shoot at Kobayashi's broken umbrella, which is caught by the wind and spins empty in the rain. At the same moment, Chianti is injured by a precise counter-shot (presumably from the real Wakasa/Asaka). RUM sees through the deception – Wakasa/Asaka used the umbrella as bait to locate and eliminate the snipers, a strategy he refers to as a "Queen's Gambit" in chess. The organization retreats. Conan, convinced that Wakasa is Asaka (among other things due to her apparent fear of water, possibly to hide scars), discusses the case with Akai Shuichi. The latter reveals that his father, an MI6 agent, was hired by Haneda's family 17 years ago to solve the murder but vanished without a trace after his departure to the USA. Most unsettling for both remains how Amanda's entire, highly skilled bodyguard team was neutralized simultaneously back then, a central unsolved mystery of the old case.