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. After further investigation, Conan tranquilizes Kogoro and reveals the murderer to be onsen manager, Koichi Maekawa. Conan reveals that Umezu was not killed by the boulder but drowned, evidenced by the dirt under Umezu's fingernails revealing the struggle. Conan explains the boulder was used to create an alibi for Maekawa by using a wooden block connected to a line and buoy. When the tide lowered, the buoy pulled the wooden block and caused the boulder to roll off.
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