The businessman Zaizen Ippei discovers a rare 20-yen gold coin from the Meiji era worth 30 million yen in an old safe at a traditional inn. During a celebration with the hostess Tominaga Kyoko, the antique dealer Kadokura, and the appraiser Kawabata, Zaizen's greedy behavior escalates as he claims the coin and suspects everyone of trying to steal it. After an argument, he retreats to the annex.

Later, a fire breaks out in the annex, and Zaizen is found dead with a head injury. The gold coin is missing. Investigators find shattered silver coins, a magnet, and traces of a glass cutter. Conan deduces that the murder occurred during the fire outbreak when everyone was in the main building.

Kogoro reveals the trick: Kadokura used the Curie temperature of nickel—he suspended a coin on a thread above a candle. When heated, the coin lost its magnetism, struck a second coin, and mimicked the clinking sound of Zaizen being alive. An ice block under the lamp delayed the fire to secure Kadokura's alibi.

Kadokura confesses out of revenge for Zaizen's fraudulent investments, which cost him his family and home. The gold coin is found inside his pocket watch. The episode ends on an ironic note as Kogoro searches in vain for valuable coins in the piggy bank.