Of all the well-worn tropes of detective fiction, the one that I have the greatest weakness for, both as a reader and a writer, is the locked room mystery.

An actual literal locked room mystery is something of a rarity. Arthur Conan Doyle’s The Speckled Band is one – a girl locks herself into her bedroom at night yet is fatally attacked, with no disturbance to the room at all. Her last, strangled words “the speckled band!” are the only clue to her killer. The Murders in the Rue Morgue is another – an elderly Frenchwoman is killed in a locked room on the fourth floor of her building (and although I don’t want to spoil either story for you, curiously the reveal is similar to the Holmes story). The novel often cited as the best locked room mystery of all time is The Hollow Man by John Dickson Carr, and that also features a literal locked room.

However more common are metaphorical locked rooms – an isolated, self-contained stage from which a small cast of characters cannot escape, limiting the solution of the mystery to a small, fixed pool of suspects. Agatha Christie wrote some of the best known in this category, including two of her finest novels, And Then There Were None, in which ten people are trapped on an isolated island and slowly picked off, and Murder on the Orient Express, where twelve passengers are trapped in a snow-bound train carriage, and the thirteenth is found dead in the morning.

My own novels betray my weakness for the genre. In a Dark, Dark Wood takes place at an isolated glass house in a remote Northumberland forest. The Woman in Cabin 10 features a literal locked cabin, but also a closed cast of suspects aboard a ferry in the North Sea. One by One follows a group of colleagues, who travel to a luxurious but remote chalet in the French alps, only to be trapped when an avalanche descends.

And now I can add to the list my own remote island mystery – One Perfect Couple – where ten strangers are taking part in a glamourous reality TV show on a luxurious but isolated tropical island. When the boat that was their lifeline is swept away in a tropical storm the contestants have to ask themselves how far they will go to survive – and the reader has to question who they can really trust.

So what is it that I love so much about locked room mysteries?

Well, part of it is the satisfaction of trying to solve the “impossible” puzzle. It’s like watching a magician pull off a conjuring trick – you’re shown something that shouldn’t be possible, a dove from thin air, a woman sawn in half, and you have to work out how it was done. That was a trope I explored in The It Girl, where April is killed in a room that only one person could have entered – and that person says he didn’t do it.
But there’s also something really satisfying about working with a small cast of characters, and having to use them effectively – and that was perhaps my favourite aspect of writing One Perfect Couple. There’s no possibility for walk on parts, or convenient extras who can provide the skills or knowledge needed for a particular reveal. You have only the characters you gave yourself at the start of the enterprise, and in the case of One Perfect Couple, that cast is being whittled down, slowly but surely.
The bonus of course is that you can get to know them really well. Characters who would otherwise have spent just a moment or two on the page can be fleshed out into real people, cropping up again and again. We get to see them change, grow, react under pressure, and come out with surprising revelations. Sometimes, of course, we see them die.
Because that’s the other thing about locked room mysteries. It’s not just a closed cast of suspects. It’s a closed cast of victims too. And when you know from the outset that one or more of that group is going to die, people you have got to know, maybe even got to like, well, that really ups the ante. And that, I think, is the real pleasure behind a book like that – the feeling that a situation has closed like a vice around these characters, with no way out. Until there is.