"I assume that something happened to Harriet on this Island," Blomkvist said, "and that the list of suspects consists of the finite number of people trapped here. A sort of locked-room mystery in island format" - page 85

The Locked-Room Trope

The locked room mystery is viewed as the perfect crime. It comes with a sense of impossibility and creates a feeling of wonder,  the thrill coming more from figuring out what happened. In that sense the focus is more on the "how" than it is on the "who". There exists satisfaction in imposing clear and concise logic to what at first appears to be acts of pure and impossible magic. The real challenge focuses on the individual's brain power as opposed to other types of mysteries that play at the human heart and spirit. 

Famous Examples of Locked-Room Mysteries:

1) The Moonstone Wilkie Collins Text

2) The Case of the Constant Suicide John Dickson Carr

3) And Then There Were None Agatha Christie

4) Suddenly at his Residence Christianna Brand

"You speak of danger. You have evidently seen more in these rooms than was visible to me."
- Sir Arthur Conan Doyle, "The Adventure of Speckled Band," 

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