After fishing, Conan and the Detective Boys are picked up at a fishing dock by Subaru Okiya while the professor is busy. On the way home, they decide to explore an island named Ikkaku Rock and come upon a dead woman's body. Conan deduces, from her wristwatch bearing her name, the woman to be Akamine Hikari, the daughter of the head of a famous company. Conan reveals that Akamine was murdered and reveals that her dying message were the names of the fishes "Mackerel, Carp, Sea Bream, and Flounder" in Kanji. Akamine's diving companions Ohto Rokusuke, Aosato Shuuhei, Kaita Yasuji, arrive on the island and reveal that Akamine went missing on their last diving trip but they thought she was okay due to her message "Take care of the rest".