(Spoiler alert: Key plot details revealed below)
Locked room mysteries - featuring seemingly impossible murders - have intrigued crime fans since the golden age of detective fiction. What's their appeal, and how many ingenious solutions can writers devise, asks Miles Jupp.
Four walls, a door, a ceiling and floor.
As crime scenes go, it doesn't seem particularly promising. Yet for over a century, some of the most ingenious detective writers in the world have been wringing suspense and excitement from locked room murders.
Last month, I found myself locked in a freezing cell in the Tower of London.
With me was Paul Doherty, a history mystery writer, who detailed the myriad ways in which he could easily kill me in just such an environment, with disconcerting relish.
Doherty, who has written more than 90 novels, calmly ran through a long list of macabre possibilities of how one might be done away with - by means of snakes or poison or even felled by arrows fired in through a slit.
The locked room genre is littered with examples of seemingly impossible murders. Perhaps a bloodied corpse is found in a room which had been locked for months. Or a victim, paranoid for his safely, concealed alone in a bank vault, is murdered nonetheless.
As crime novels go, such stories are a world away from contemporary, grimy police procedurals or Ruth Rendell-style tales of psychopaths and loners. Locked room murder mysteries do not aim to shine a light on the darker and more brutal realities of our existence. Each one is a puzzle.
This is a world in which detectives, often posh or whimsical, grapple to solve crimes that should not have been achievable in the first place.
Writers of locked room mysteries are not interested in the psychology of the killer, or the drink problem of the detective. What fascinates them is the thrill of setting up a fiendish crime, and challenging the reader to solve it.
Mild-mannered Robert Adey has an encyclopaedic knowledge of the locked room murder genre. In his 1991 bibliography he lists more than 2,000 of them, and at home has a bulging file bringing his research up to date.
Within the covers of his innocent-looking book is a resource that could launch the career of any would-be crime novelist, or indeed serial killer. The volume contains page after page describing bodies found dead in castles, lighthouses, submarines and deserted houses.
What they have in common is that their being killed should have been inconceivable. And yet they are all stone dead. (Apart from a very few who were pretending in order that they might fool a detective or be switched at the last minute for the body of their identical twin brother. Or a waxwork - over the years these writers have tried almost everything.)
But if the list of puzzles at the front of the book is intriguing, it is at the back of Adey's book where things get even more interesting. Here one can peruse the solutions to these crimes which have bamboozled the most wily of readers and tested the powers of the most perceptive of detectives.
Within this list of solutions can be found bats who dislodge ceremonial daggers so that they plunge into the heart of the victim, or vicious cats whose claws have been dipped in poison, sliding doors, hidden panels and gas-filled glass vials crushed under heel. Each one reveals something of the extraordinary ingenuity of writers of locked room mysteries.
And when it came to ingenuity, none was more prolific or more imaginative than John Dickson Carr.
Dickson Carr, who also wrote under the pseudonym Carter Dickson, was a genius of the locked room genre, a man able to wring mind-bending mysteries out of a bloodied corpse found in the most sparse of surroundings.
In novels like The Crooked Hinge, The Hollow Man and Castle Skull, readers thrilled to tales of men shot dead in closed elevators, strangled to death in locked huts and abducted from railway carriages under constant surveillance.
In 1981, The Hollow Man was selected as the best locked room mystery of all time by a panel of 17 mystery authors and reviewers. At least there were 17 when they went into the room...
Today, his novels still give enormous pleasure. But not every locked room mystery lives up to his standard. The plots of some of these books creak like ancient stage machinery. Dwarves are concealed in tea chests and Ming vases to leap out and kill the victim.
Identical twin brothers appear from nowhere and psychotic badgers are trained to leap out of their setts, armed to the snout with semtex. (I have only made one of these up.)
No such barrel-scraping occurs in the work of the man who has single-handedly led a renaissance of the locked room mystery in his native France.
Anglophile Paul Halter sets his ingenious mysteries in an England he fell in love with through the books of Agatha Christie and Arthur Conan Doyle. Today he has written more than 30 novels, each one wrapped up with a truly inspired solution.
So could the locked room mystery stage a similar comeback in Britain? Not necessarily, according to publisher Daniel Mallory of Sphere, who consigns the locked room mystery to the world of the "cosy" crime thriller.
"If someone sent me a modern one though I would read it, mostly because I'm very slow and never guess the solutions."
But for any reader really wanting to immerse themselves in the pleasures of the locked room mystery, I have one piece of advice - look east. In the land of Sudoku, Japanese readers thrill to the pleasures of impossible crimes.
The godfather of the genre is Soji Shimada, whose Tokyo Zodiac Murders combine a tale of a serial killer, an astrology obsessed detective, and a puzzle mystery. It is gripping, frightening stuff.
So while some of the stars of the stories may have gone the way of the body in the locked library, if you're prepared to go looking for it, this old-fashioned genre remains alive and well.
You'll never look at four walls, a ceiling and a floor in the same way again.