Heiji invites Conan, Ran and Kogoro to join him and Kazuha on a case, he got a letter about. The letter is from a woman named Saori, who lives on an island known as the mermaid island. The island is famous due to it's elder who is over 100 years old and rumored to be immortal. Apparently Saori is afraid of getting killed by mermaids. Conan isn't pleased with being dragged on this case, but Heiji reveals to him that while the letter was sent to his address it was meant for Shinichi. After arriving on the island, the group learns that Saori has disappeared three days ago. No one is worry, because it's not unusual for Saori to leave home after having a fight with her father. They meet four of Saori's childhood friend, Naoko, a shop clerk, Toshimi and her fiancée Rokuro and Kimie, a shrine maiden who is also the great-granddaughter of the elder. Each year there is a festival on the island where the elder awards three winners with a dungong arrow, that wields protection for it's bearer. Saori won one last year, but lost it last week, and up to her disappearance, she had become fearful of being cursed of mermaids. Kimie reveals that last time she saw Saori was three days ago. Kimie needed to go the dentist on the coast, and Saori went with her. She promise the group to take them to Saori's house after the festival, and offers Ran and Kazuha each a ticket to the lottery for the Dungong Arrow. She had two left after a couple cancelled. During the festival the Elder reveals the winning numbers, and Kazuha's number is one of the three. Later at the mermaid waterfall, Kazuha and the two other winners, Naoko and Saori's father, receives their arrows, but as the firework is shoot up, the audience sees a body hanging in the waterfall.
