The Disappearing Fish at Ikkaku Rock (Part 1)

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.

Synopsis excerpt via Kitsu.