Ran's karate senpai, Kyodo Nanami, invites the Detective Boys for a treasure hunt at her home. Once there, they discover the fact that the treasure is very likely hidden in the Dark Tower. However, there aren't any entrances or windows to enter the tower with. Around the estate, they also discover that this house has been equipped with various tricks and traps. Later, Nanami's mother, Kyodo Momoe, had been discovered dead, thought to have fallen from the tower.