A young and pretty woman named Mitsuko is found by a reef in what appears to be a suicide. When information that the town's doctor had been intimate with her in the past is revealed, and Mitsuko's best friend Tomoko insists it was murder, both Conan and Kogoro suspect she is right and that killing is really involved. Now the two must find the clues before his drinking alibi fully comes through... but what to do when Tanaka was actually drinking with Kogoro himself?
Tomoko had tearfully said that she suspected Dr. Tanaka of being the killer, and she was right. Tanaka killed Mitsuko since she was his girlfriend at the time, but he had been offered a very good arranged marriage to his boss's beautiful daughter. Thinking that Mitsuko's presence would ruin an alliance that would further his career, he decided to get her out of the way definitely.
Tanaka brought a floating ring and hid it, then drugged Mitsuko and took her to the seaside. He then tied up a boulder to her bare feet, put the floating rin around her and left her ashore. The waves took Mitsuko inside, and soon she was at the bottom of the sea, unable to escape and ultimately drowning. Meanwhile, Tanaka was out drinking with others, and this weas his alibi.
