Professor Agasa takes the Detective Boys camping, but he forgets the tent! As they look for shelter, they wander into a castle. Inside, Conan finds a hidden passageway, but is knocked out by someone hiding inside! Professor Agasa and Haibara realize something has happened to him when they cannot find him.

The Detective Boys attend a triple-feature of Gomera during the final day of a theatre before it is destroyed. During the second film, the man is found hanged. The four theater employees become the main suspects.