Conan, Ran, The Detective Boys, Haibara, and Kogoro go on a trip to Mt. Aso. They meet a boy called Justin and his mother there. Justin shows them his stamp collection book, where he has stamps from all the tourist attractions at Mt Aso. They later part. While they are on a break, we see a man leaving behind a bag on a bench, and another man picks it up and leaves behind a stuffed toy. Ayumi tries to return the toy but is pushed away by the man. He is chased by the police and Conan. They see the man dashes into Justin, who is carrying a similar toy and the toys get exchanged. Everyone returns and we find that the man who left the bag is Monkichi Kumada, who owns Kumamon Foods. He says a thief stole important details regarding new sweet tastes which are un-patented and demanded 10 million. Another detective calls, Kouda, and informs them that the thief is caught but there was nothing in the stuffed toy. Conan explains that the toys got exchanged and tracks down Justin's location.