The Detective Boys are on a camping trip but Professor Agasa forgot the tent. They then wander to a castle owned by an old mistress and learn that there's a secret treasure in the castle. Conan finds a clue to the treasure and enters a secret passage but is attacked by an unknown assailant. Agasa and Ai realize that something has happened to Conan. Agasa is about to phone the police to investigate but upon seeing a secret agent, is knocked out by the same culprit.