Dickens's last novel is fiction's equivalent of Fermat's Last Theorem - that fiendish mathematical problem the great 17th-century physicist left for posterity to solve. The full title of Dickens's last novel is The Mystery of Edwin Drood. Mystery is the key word. No doubt viewers will be entranced by the BBC's two-parter. But Dickensians will be watching with a sharper curiosity this dramatisation by Gwyneth Hughes, who's also completed the narrative. "Has she cracked the problem?" they'll ask.
Drood is a whodunnit. But there's no need to shout "spoiler alert!" since Dickens died halfway through writing, and kept his cards very close to his waistcoast. (He loved fancy waistcoats.) We can only speculate what denouement he had in mind. Hughes may have ended that speculation, but could the text of Edwin Drood itself also hold the key to another mystery and unlock the dark secrets about the death of one of our most feted writers?
The "Great Inimitable" (which is what Dickens called himself) left us half a story. The action opens in, of all places, a sleazy London opium den, where a man is hazily coming round, surrounded by sleeping Oriental addicts and lascars (seamen from the East Indies).
He is, we observe, not one of the scum who are regulars of Princess Puffer's evil garret, but a "gentleman". A clergyman, no less - and, as the world wrongly thinks, an eminently respectable man of the cloth. John Jasper is the precentor (choirmaster) at Cloisterham Cathedral. It's recognisably Rochester Cathedral, just down the road from the fine house Dickens lived in, Gad's Hill Place in Kent.
Dickens apologises, in his novel, for the Cloisterham pseudonym - but it's understandable. The authorities in the place where he himself worshipped on a Sunday might not have appreciated the world being informed that they were in the habit of appointing junkies to lead the choir.
Dickens liked teasingly complex plots in his late career and none is more complex than Drood. But at its centre is a familiar crime scenario involving sex, money and homicide. Jasper, we learn, is uncle and guardian to an orphaned young man coming into his majority (21 years of age). Edwin Drood has what Dickens elsewhere calls "great expectations" - a substantial cash inheritance. But there's a catch. He will (as their respective parents agreed before they died) only inherit if he marries another orphan, Rosa Bud.
Jasper secretly lusts after Rosa, who is still at school in Cloisterham, under his tutorship. The name Rosa Bud (her nicknames are Pussy and Rosebud) is straight from the pages of Victorian pornography - as gentleman readers of the time would chortlingly have realised. Dickens himself had disposed very cruelly of Mrs Dickens, the mother of his ten children, ten years earlier in favour of an actress, Ellen Ternan, almost 20 years his junior. This is a section of Drood that vibrates interestingly with the author's and his age's complicated sexuality.
The plot is thickened by the arrival of twins, Neville and Helena Landless, who have come from Ceylon to finish their education at Cloisterham. They may or may not be of mixed race. Ever since the Indian Mutiny of 1857 - in which he favoured mass execution for the rebels - Dickens had had strong feelings about the subcontinent.
Edwin goes missing after a violent storm. The male twin, Neville, is suspected by the police of having killed him - he, too, has designs on Rosa. Jasper is suspected by the reader who knows, as the police do not, about his opium habit. But since Dickens inconveniently died at this point in the narrative, we can't get beyond suspicion. Drood remains a mystery wreathed in mystery.
How did Dickens intend it to end? He normally jotted down plans but there are, oddly, none left for the second half of this narrative. He hinted to his future biographer, John Forster, that Jasper killed his nephew Drood, and buried the body in the crypt of the cathedral, the easier to have his filthy way with Rosa.
But the hint Dickens gave Forster is vague. He loved bringing characters thought dead back to life, as in Our Mutual Friend where the hero is drowned - but then turns out not to be drowned after all. Was Edwin similarly to reappear? One of the titles Dickens toyed with for the novel was Dead? or Alive? Which is Edwin?
A mysterious character, Dick Datchery, makes an appearance in the last chapters Dickens left us. He's clearly someone in disguise - but who? Edwin? These are questions to which Dickens leaves no answer.
How, one wonders, will Gwyneth Hughes solve the puzzle? But there is another mystery shrouding Drood.
By the time he wrote the novel, Dickens was well into what critics call his dark period. No work is more morbid. Dickens had actually looked death in the face five years earlier. When he was return ing from France on 9 June 1865 with his mistress, Ellen, the train hurtled off the rails at Staplehurst in Kent. The carnage was terrible. Dickens behaved heroically - saving lives and (as an afterthought) that month's manuscript of Our Mutual Friend, the novel he was writing at the time.
He survived but never recovered. His daughter Mamie testifies that Staplehurst hastened his death, and he upped his intake of laudanum (opium mixed with alcohol - a favourite Victorian cocktail). He observed that his watch (a "special chronometer") never kept the time again.
Dickens, the biographies tell us, suffered a catastrophic stroke on the evening of 8 June 1870 while dining at home. He'd spent the day writing Chapter 23 of Drood. He collapsed over the table. His last, incoherent, words were "on the ground". He never regained consciousness and died after a 24-hour coma.
But there's a possibility that Dickens killed himself with an overdose of the laudanum. Some might say that's far-fetched; nonetheless it appeals to me.
There are four tantalising hints:
(1) Drood was scheduled to run to 12 monthly numbers. It stops precisely at the end of the sixth. It's as if Dickens planned this premature termination at the halfway point to leave posterity forever guessing.
(2) Dickens was using laudanum and had an unlimited supply. Overdose causes symptomless, painless death.
(3) Dickens requested that he be buried in Rochester ("Cloisterham") Cathedral where, it is conjectured, Edwin was buried. The nation demanded Dickens be buried in Poets' Corner in Westminster Abbey. Rochester has, from time to time, requested the body be exhumed and returned to where he wanted to be. The request, unsurprisingly, has been denied.
(4) And on which date did Dickens die..? 9 June. The same date as the Staplehurst disaster five years earlier. Coincidence? I'd love to think that Dickens, who knew it was coming, composed his death as carefully as he composed his fiction, intending to leave posterity forever wondering and speculating.
It took physicists 358 years to crack Fermat's Last Theorem. Let's hope Gwyneth Hughes cracks the Drood conundrum after a mere 142 years, as Britain celebrates the 200th anniversary of the Great Inimitable's birth.
This is an edited version of an article from the issue of Radio Times magazine that went on sale on 3 January.
The Mystery of Edwin Drood begins 8pm, Tuesday 10 January on BBC2.
The Dickens Dictionary by John Sutherland will be published on 2 February. His most recent book, Lives of the Novelists: a History of Fiction in 294 Lives, is available now, published by Profile.