In a “locked room” mystery, a murder (or other crime) happens in a room that seems to be completely, well, locked to the outside world; typically from the inside. So in order to solve the crime, the detective has to figure out not only who committed it but also why the scene of the crime is a locked room in the first place, and how the perpetrator managed to get in and out of the room and leave it behind apparently locked from the inside. — Sometimes, the locked room is also used as a decoy or in order to provide the murderer with an apparently unbreakable alibi for a crime committed somewhere else.
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