Sherlock, Series 2 Finale, BBC One

And so we reached the end of series two with The Reichenbach Fall, the last of a miserly three episodes.

I suppose this criminally meagre ration leaves us eager for more, though the way Benedict Cumberbatch's career is rocketing skywards and Hollywood-wards, it might have been wise to shoot some more episodes with him while he still had the overcoat on and the violin to hand.

Anyway, Moriarty was back, played once again with a kind of amphibian slipperiness by Andrew Scott (pictured right). This week's screenwriter was Steve Thompson, who had been set the daunting task of constructing an escalating battle of puzzles, bluffs and conundrums as Moriarty sought to lure Holmes onto the battlefield and then destroy him by means of his own cleverness. Moriarty's cunning plan was to paint Holmes as a fraud, and as Sherlock himself commented, the fiendish part was that it was "a lie all wanted to believe". Many, in other words, would prefer to believe that Holmes was a cunning con-man rather than a far-too-clever genius of detection, not least those who had been made to look foolish by his brilliance. This would include the press, incarnated here by the ambitious and unscrupulous tabloid hackette Kitty Riley (Katherine Parkinson, pictured below left). 

I felt the piece flagged a bit with about 20 minutes to go, as if everybody was treading water while the writer racked his brains about which rabbit hole to dive down, but eventually he came up with a cracking conclusion which left you with a lump in your throat while scratching your head. Once again the story had been devised as a set of variations on a Conan Doyle original (The Final Problem, in which Holmes and Moriarty fought to the death). Although there was no trip to Switzerland, we had a final plunge which might not have been all that final, as well as a false identity for Moriarty which translated as Reichenbach (Rich Brook). 

Among other pleasures, there was some superb comic writing. The opening scenes had been devised as a montage of Holmes being acclaimed for cracking assorted cases, and receiving gifts from grateful clients. He was handed a package for retrieving a Turner painting of the Reichenbach falls. "Diamond cufflinks," grunted Holmes irascibly, without bothering to tear open the elegant wrapping. "All my cuffs have buttons." Next up was a tie pin ("I don't wear ties"), then the coup-de-grâce from Inspector Lestrade (Rupert Graves, pictured below) and his police comrades - a deerstalker hat. Cumberbatch's expression of lemon-sucking contempt as he was forced to model the headgear in front of the cheering constabulary will surely become a cherished YouTube moment.

The theme of Holmes being too clever for his own good resounded loud and clear through the scene where the Great Detective was called as a witness in Moriarty's trial at the Old Bailey. The smirking Mephistopheles of Crime had broken into the Bank of England, Pentonville prison and the vault housing the Crown Jewels simultaneously, though curiously hadn't stolen anything. Odder still, he offered no defence.

Called to the stand, Holmes patronisingly lectured his barrister on what questions she should and shouldn't ask him ("no, don't ask that, it'll be overruled... that's really not your best line of questioning" etc), then delivered a swift analysis of the jury members after one sweep of the benches with his hi-def zoom eyesight. "The foreman's a medical secretary, trained abroad judging by her shorthand," he announced. "Two jurors are having an affair, with each other it would seem..." At this point he was bawled out by the judge, who couldn't tolerate being upstaged in his own courtroom.

Between the seesawing poles of hubris and nemesis stands the doughty Dr John Watson (Martin Freeman), who has the moral conscience and straight bat of an upstanding Edwardian cricketer, even if you still can't help thinking of him as that guy from The Office. Watson is forever whispering rational advice in Holmes's ear which could, if followed, make him appear a little more like a functioning member of society rather than a sociopathic pariah with a God complex, but if Holmes wasn't arrogant, abrasive and egomaniacal we might as well all give up and watch Inspector Lynley. Nor would he have his very own super-villain to keep him up to scratch. As Moriarty himself put it, "every fairytale needs a good old-fashioned villain". (Note to producers: isn't there a job for Mark Strong here somewhere?)

There have been moments in this micro-series where we seemed to be straying dangerously close to the perimeter wire of Doctor Who and its penchant for clever self-indulgence. Tonight's Sherlock slithered towards the precipice with its silly repeating gag about sinister foreign assassins bumping each other off, as if we were suffering interference from a parallel Clouseau world. 

However, it's the core duo of Holmes and Watson which has been the show's great triumph, regardless of the company and plotlines they may be keeping, with Freeman's long-suffering, somewhat muleish contribution as vital to the delicate balance as Cumberbatch's flaring-nostrilled steeplechaser. In episode two, The Hounds of Baskerville, Holmes was extolling the value of having Watson's mediocre intellect alongside to help lift his own to even greater heights, blithely indifferent to Watson's miffed reactions. This time, we were allowed to feel a real depth of emotion in their relationship, Watson even being moved to confess that "I was so alone and I owe you so much," although admittedly at that point he thought Holmes was dead. There was room for a bit of fun with the "are they gay?" theme too, as Watson read indignantly from a tabloid news story: "'Bachelor John Watson' - what the hell are they implying? 'Confirmed bachelor John Watson'!!..."

Love of any denomination remains a distant afterthought in Sherlock's inner labyrinth, though there was the briefest of moments with lab technician Molly (Louise Brealy), when Holmes dropped his guard and allowed an expression of profound tenderness to steal across his face. You could actually see the process happening in Cumberbatch's eyes. It's at times like this that Sherlock looks like a very superior product indeed.

• Sherlock: The Six Thatchers reviewed on theartsdesk