On a hot summer night, Homura Roku came to the Mahjong Club to relax. The club was overcrowded and he joined Hoshio, Matsuyama, Kawagaoka Midori and Sonobe in the mahjong game. During this period, Matsuyama died suddenly, and Fancun quickly launched an investigation. He found a scar on Matsuyama's thumb and suspected it was related to poisoning. By examining the mahjong table, Homura deduced that the prisoner killed Matsuyama with mahjong tiles coated with poison. During the investigation, absorbent cotton became a key clue, and all parties insisted on its origin. Homura continues to reason and gradually locates Sonobe as a suspect. How will he respond? The case becomes a mystery.
