137화: 아이바라가 탐정단에게 성에서 무츠미 니시카와의 10년 된 살인 계획을 밝혀내라고 이끈다.

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Haibara leads the investigation and is accompanied by the Detective Boys. They enter the same secret passage as Conan did and wander out of another one. Genta and Mitsuhiko are captured by the culprit. Haibara and Ayumi continue further into the abandoned tower and discover the corpse belonging to the grand mistress. After a lengthy chase, Haibara puts the pieces together and reveals the culprit to be Mutsumi Nishikawa. She murdered Mrs. Mamiya, the others, faked her death, underwent plastic surgery, and assumed Masuyo's identity after finding her dead, just to search for the hidden treasure in the castle. Mutsumi grabs the girls and tries to kill them. Conan, Mitsuhiko, Dr. Agasa, and Genta appear. After reuniting with everyone, Conan reveals the location of the treasure, which literally is the castle holding a beautiful scene over the valley. Mutsumi, completely devastated and realizing the time and magnitude of her crimes, comes into terms that she wasted 10 years and killed a huge number of people just for this. She is later arrested. (Source: Wikipedia)