Mystery Case
Noun
かいじけん
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.
A suspicious incident (事件) always has a strange mystery behind it. It also always turns into a mystery case.
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.