Can The Mystery of Edwin Drood ever be truly solved?
Only its author, Charles Dickens, knew how the story was meant to end and his death in 1870 put paid to any hopes of readers ever knowing for certain what its outcome was supposed to be.
Now, a major new BBC adaptation of Dickens's final work attempts to solve the unsolvable.
Gwyneth Hughes, the West Yorkshire author behind this latest version of Drood, said the project was "a mammoth task", adding: "It nearly killed me on occasion."
Ms Hughes, from Slaithwaite, said she did not realise the enormity of what she had taken on when her plan to finish The Mystery of Edwin Drood was first given the go-ahead by the BBC.
"At the time, I was terribly excited about this marvellous new job," she said.
"But, as I got into it, I just began to realise the Himalayan scale of the task ahead of me. It was very daunting.
"This is half of a book by our greatest novelist."
Ms Hughes is not the first person to try to satisfactorily conclude this 152-year-old tale of drug addiction and sexual obsession.
However, she said she hoped her version would entrance newcomers as much as Dickens's most stalwart fans.
The latter, she admitted, can be hard to please.
"It's difficult enough putting novels on the telly when you have to leave a lot out and you annoy a lot of the fans," she said.
"Now, here was I proposing not only to leave a lot out, but to put a lot in."
Dealing with the story of drug-addicted choirmaster John Jasper, The Mystery of Edwin Drood is a "dark and gothic" tale which is "very spooky, scary and modern", she said.
Faced with the task of finishing a story which Dickens began just over a century and a half before, Ms Hughes said she faced working with a plethora of "fantastically vivid characters".
"I enjoyed their company even as I wrestled with the hideous hell of making a plot that worked," she said.
"What I've done is to concentrate on the characters at the centre of the drama. It's Dickens, so there are tons of people in it who are all being funny and grotesque.
"In a thriller like this they don't really work so, with huge apologies to fans of the book, I really concentrate on the central character."
She said that, though she might think twice before taking on such a task again, the whole process of finally solving a classic Victorian mystery had been "great fun".
She said finishing The Mystery Of Edwin Drood had only confirmed her admiration for Dickens.
"The more I worked on it, the more of a fan I became," she said.
The first part of The Mystery of Edwin Drood is shown on BBC Two at 21:00 GMT on 10 January and concludes at the same time the following night.