Supermarket Beikado employee Kanzaki receives death threats over the phone. He hires Kogoro to investigate the threats, but Kogoro believes it to be a prank played by a friend. However, an incident occurs the following day on Sunday. A man wearing a ski mask appears at Beikado. While shouting, "Die, Kanzaki," he stabs the man dressed in Beikado's mascot Bekapon costume and runs away. The victim is employee Shimomura. Every Sunday, Kanzaki interacts with customers dressed as Bekapon, but due to a stomach ache, Shimomura wore the costume in his place. Megure believes the culprit accidentally stabbed Shimomura, but Conan notices something puzzling about the case...