Kogoro takes Ran and Conan to an onsen by the ocean. When they hear the sound of a boulder falling, they find a man named Hideaki Umezu, a mortgage company president, dead. The police presume that a boulder rolled off the hill and killed Umezu. However, Conan reveals that there are scratch marks around the spot where the bolder rested revealing that someone used the boulder to murder Umezu. The suspects are Umezu's secretary, his two daughters, and the onsen manager.