After the sublimely byzantine A Scandal in Belgravia and the captivating chill-a-thon of The Hounds of Baskerville, it was always going to be difficult for The Reichenbach Fall to match the high standards set by its predecessors.

Furthermore, in the first series, the most ordinary of the three amazing stories – which is rather like saying one was ‘only’ Spiderman while the others were Batman and Superman – was Steve Thompson’s The Blind Banker. It was great but it wasn’t quite exceptional, so we approached his second story for the programme more in hope than in expectation – the people who make Sherlock are only human, after all. It’s unfair to demand the magnificent every time … isn’t it?

No, actually. It isn’t. Because The Reichenbach Fall is not just magnificent, not just the best of the series (and thus the best episode of the show so far) and not just most likely the best thing you’ll see on the telly this year.

As Sherlock himself remarks, ‘It’s a game changer.’ It’s a smack in the face, heart and groin all at once that doesn’t just change the game and move things to a new level. It invents new levels to match its own astonishing scale of ambition.

The loose theme of this second series has been the humanising of Holmes: turning a finely-tuned detecting machine into a person. Having toyed with feelings (both Irene Adler’s and his own) in the Episode 1, and discovered the perils of self-doubt in Episode 2, The Reichenbach Fall sees Sherlock put through what is, for him, an emotional-wringer of Olympian proportions.

Having become a tabloid sensation in a deerstalker (‘It’s an ear hat!’) the detective is brought crashing down by the machinations of Rossini-loving rogue James Moriarty: first humiliated in the press, then arrested as the architect of all the cases he supposedly solved.

‘Every fairytale needs a good, old-fashioned villain,’ the consultant criminal coos, and his coup de grâce is forcing his nemesis to commit suicide in order to save the lives of the people he cares about.

John Watson, Mrs Hudson and Inspector Lestrade will all be murdered if Sherlock doesn’t top himself, and not even Moriarty’s death will stop the hitmen carrying out their assignments. It’s about as bad a time as possible to discover one has the capacity for such boring, human things as friendship – particularly when the good, old-fashioned villain blows his own brains out. The only way of saving the day is to follow suit. ‘Goodbye, John,’ Sherlock whispers, and leaps off the roof of Barts Hospital.

The detective’s journey from robotic genius to human being is beautifully played by Benedict Cumberbatch, and one would have to be a hardhearted Holmes-hater not to be moved by his tearful depiction of Sherlock’s last phone call: the false confession of fraudulence and final farewell to his one true friend before his jump into oblivion.

But it’s afterwards, when the blood has been scoured from the sidewalk and 221B Baker Street has been cleared of the detective’s possessions, that the emotional floodgates are really forced open – and it’s all down to Martin Freeman’s astonishing performance.

‘I was so alone … and I owe you so much,’ John Watson says, his weary heartbreak reflected in the black onyx sheen of Sherlock Holmes’s grave. It’s as sad, helpless, rueful and true a portrayal of grief and loss as we’ve seen for ages, and it’s an all-time highpoint for the series so far.

Hell, it’s an all-time high for television – and there’s still time for the revelation that Sherlock isn’t dead after all before the drums kick in and the credits roll. Phew. We needed a few moments before we could even move, let alone think about what might come next.

However, it’s something to consider when calm is restored and brain activity resumes: where does Sherlock go from here? How can it surpass an episode – a series – of such supreme quality and success?

That there will be a third series of the show must surely be beyond doubt; not just because of the unresolved strands of the story (is Moriarty dead? A tenner says that the slippery, self-styled Mr Sex can survive a bullet through the brain) but because the BBC almost certainly won’t allow such a runaway triumph to come to a halt yet.

But how can Mark Gatiss and Steven Moffat outdo something so untoppable? Is this Exile on Main Street or is it only Sticky Fingers? Are there even greater things yet to come?

Aired at 9pm on Sunday 15th January 2012 on BBC One.

> Buy the Series 2 DVD on Amazon.

> Buy the Series 1 DVD on Amazon.

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