Conan finds Hiramune murdered with the letter "Z" burned into his back. The police department arrive and are confused as the letter they were expecting was "N" as the previous victims had the letters "E", "S", "W" and were thought to be Cardinal direction. They notice the Hiramune's left hand was on his keyboard on the keys "Ctrl" and "C" and his right hand holding the wire of his mouse. Takagi and the Junior Detective League decide to accompany him and his interrogation of the victim's relatives. Conan than realizes the victims share one thing in connection, Mahjong.

The Detective Boys decide investigate spooky Ch'teau when a man starts to live there. They find the ch'teau and its garden have been remodelled. The next day, the man living in the château, Rokuro Wada is revealed to have died. They learn from Keiichi Yagami, Wada's agent who sells his paintings, that Wada had been working on a masterpiece at the ch'teau for three months. They decide to search the ch'teau for the painting but to no avail. Afterwards they are visited by Wada's old aunt Saeko Takatoo and her caretaker Mina Eguichi who reveals that Wada sent them a letter telling them to visit the ch'teau tomorrow to view the masterpiece. Eguichi reveals Wada's masterpiece is going to help Takatoo regain her memories which she lost from an accident a year ago. Conan finds a photo revealing a ch'teau Takatoo owned in the past and realizes what Wada's masterpiece is. Conan reveals the masterpiece would be complete tomorrow. The next day, the flowers at the ch'eau have bloomed revealing a scenery similar to the one in the photo. The scenery causes Takatoo to regain her memories of her life.