Strange Mystery
Mystery Case
noun
A suspicious incident (事件) always has a strange mystery behind it. It also always turns into a mystery case.
かいじけん
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
ホームズでさえもその怪事件の糸口を見つけることはできないだろう。
Even Holmes wouldn’t be able to find a clue to that mystery.