The Case
Conan along with Heiji, Ran, and Kogoro visit the Isle of Mermaids looking for Saori Kadowaki, who had sent a letter to Heiji asking them to save her from being killed by a mermaid. In secret Heiji confines to Conan that the letter was meant for Shinichi despite being send to Heiji's address. But when they try to look for Saori she has been reported missing. They decided to talk with Saori's childhood friend Kimie, who is the elder's great-granddaughter. Rokuro Fukuyama the fisherman is Toshimi's fiancé. Those two along with Naoko, a shop clerk, are also childhood friends with Saori. They don't seem concerned over Saori's disappearance, but that's because Saori often left home, when she had argued with her father. Every year on this island a festival is held where three people win the Dugong Arrow that is said to wield immortality to its carrier. Saori won one last year, but have lost it and had since been paranoid about being curse by mermaids up until her disappearance. Kimie offer Ran and Kazuha each a ticket to the lottery, since a couple called off, and promise to take them to Saori's house after the festival. During the festival Kazuha wins an arrow, which she receives along with Naoko and Saori's father Benzo. The festival is brought to a disastrous end when one of the winners is found dead hanging down the waterfall.
The deduction
Conan and Heiji investigate the area and history of Mermaids Grave, a forest on mermaid island. They learn that an elder who is over 130 years old is a holder of the arrow and the head of the festival. Three years ago a warehouse got burned down and a half body was discovered missing its legs. The police from the coast said that the body was of a middle age woman, and concluded that it might have been a tourist, who snuck into the warehouse to look for a dugong arrow and accidently started the fire. The deceased was never identified. But rumors had been going around that it was a body of a mermaid. The year after the incident, the bones were buried on the island, but later moved into the forest on orders from the Elder due to grave robbers who had stolen bones from the grave. In myth it was believed that bones of a mermaid grant long life. It was that incident that made the Elder famous around the country. Naoko suggest that Toshimi had been looking for the grave, and Kimie confirms that Toshimi often spoke of it. While investigating two more murders occur. First Naoko being strangled and her arrow is missing. The only connection between the three women, beside being childhood friends, is that they all three believed in the power of the arrow, and that the Elder was immortal. Heiji and Conan decides to take a look at the book with the names and numbers for those who anticipated in the festival's lottery, but the most recent one is missing. While Kimie goes out to search for it. Heiji and Conan takes a look at the old ones. Conan finds a name, Shihi Miyano, and remembers that it was Haibara's real name, but doubts that's her. Then Kimie gets burned in the warehouse near the shrine. After the corpse's identity is determent by dental records, Rokuro breaks down crying and the 'Elder' takes to her bed muttering about seeing young life dying, while she still lives. The group splits up, Heiji and Kazuha goes out searching for Benzo, while Rokuro takes the rest to Saori's house, so Ran can see a picture of Saori and determent if she was the woman, she and Kazuha saw. After searching a bag for travel in the house Conan realizes that something is missing. He reveals to Heiji he knows who the murderer is, relay his theory, and asks him to meet up later, but later Heiji and Kazuha fall off a cliff after they discovered the much-talked-about grave. At the grave Heiji sees that someone has visited it recently and begins to wonder if Conan's solution was correct.
The Resolution
Heiji saves Kazuha and they're still having trouble. While everyone is waiting for the deduction of Sleeping Kogoro, Ran arrives with Lady Mikoto, to the chock of the island's elder citizens. Sleeping Kogoro is now going to present a deduction.