This piece contains a number of very significant spoilers about the BBC Sherlock Holmes episode ‘The Six Thatchers’, and other earlier episodes. You have been warned.

You might have heard of a fictional detective called Sherlock Holmes. I understand the stories written about him and his friend Dr Watson were very popular in the late Victorian era, and continued to be so, well into more recent times. The formula of a brilliant detective who is also something of a monster, and only by the slimmest of margins qualifies as morally better than the bad-uns he is seeking to defeat is now almost universal in the detective fiction genre, of which Holmes’ creator was one of the first exponents. It’s true that Maigret and Miss Marple are exceptions, but in the broader field of film and TV detectives, it’s very rare to find a successful detective who isn’t some kind of ‘maverick’, perpetually close to being fired for malpractice – which in the case of Dirty Harry, for example, could involve shooting people. Also common is the presence of a lolloping sidekick who struggles to keep up with the machinations of the genius’s mind to the huge convenience of the audience who thus had some hope of knowing what the hell was going on.

What a stroke of genius then, to update the original Holmes, put him in a flat over a shop in modern Baker Street, cast a brilliant but not that well-known actor to play the role, and have all the old trappings brought into the twenty-first century. Make it all smart phones, blogs and text messages, ingenious visuals to flag Holmes’ famous powers of observation, and then you’ve even got time to have amusing postmodern references to the old stories by bringing in a deer-stalker or two, or even setting an episode back in the nineteenth century and then revealing that it was actually all a dream.

Stroke of genius it was, and the franchise has been extremely successful, and made the actors involved world famous. Everyone loves it, and each new series (or one one-off special) gets pride of place in BBC’s listings in the post-Christmas schedules.

Almost inevitably, however, it has over-reached itself, or ‘jumped the shark’, in the parlance of TV pundits. It was already looking like it was about to start climbing up its own rectum several episodes ago, but since the gap between them has been so large this wasn’t as apparent as it has now become.

The latest episode has a plot which can be summarised as follows: Someone is smashing statues of Margaret Thatcher. Why? Because they are trying to find a USB stick which they know to be hidden in one of them. How did this get there? It was put there in a factory in Tbilisi by a mercenary on a mission. This mercenary believes that Mary, Doctor Watson’s wife, who is also a mercenary betrayed him, and now wants to kill her. Instead he gets killed, and it turns out that the person who in fact betrayed her was a little old lady from the first scene who made a joke about mivvis that threw you off the scent. In an showdown implausibly taking place in the London Aquarium (an excuse for earlier moody scenes of sharks) she pulls a gun and shoots Holmes, but Mary dives in front of him to take the bullet and dies. Watson, unsurprisingly, is pissed off with Holmes.

Okay, so what’s wrong with that, you may ask. It’s Sherlock Holmes for goodness sake, crazy stuff happens. Well, you may ask that, but I won’t agree with you. In general, the whole point of a detective story is that it makes sense, and that all the loose ends tie up and are explained. A ‘high-functioning sociopath’ obsessed with logic and rationality demands no less (that’s Holmes, not me). Ideally, the ‘reveal’ should be ingenious, something which the author has waved in front of our faces so that we feel that we could, if we had been more on the ball, solved the case ourselves. Ideally, I maintain, the whole plot should be free-standing and internally consistent. This is one of the ways Sherlock has lost it. It has become pre-occupied with an increasingly outlandish back story and story arc, the heavy price of having constantly to come up with sensations to keep the ratings up. Deaths keep getting faked, people keep getting fatally shot without dying, Moriarty keeps reappearing inexplicably – even though he appears in only one original Sherlock Holmes story, the same one in which he dies – and coincidences escalate.

Personally, I can pinpoint the precise moment when I wanted to throw the remote across the room. It occurs just after Holmes looks down with horror at the USB stick in the shards of the Thatcher bust he has just smashed and we go into a flashback of the preposterous back-story, which I had mercifully forgotten, that Mary Watson (who scarcely gets a mention in Conan Doyle) had a secret past as an assassin that would shock John so much that if he knew its details he would stop loving her, all of which details were on the stick that she gave him and which he destroyed so that he could continue in his love.

Every one of the four assassins, it turns out, could trust each other because they each had an identical copy of the USB stick with which they could betray them all. Yes, read that again. It’s not quite how trust works, as far as I know. So why did he hide the USB stick? Oh, because he could see he was about to get caught and interrogated, so it was a precaution. You see, if you’re a mercenary, and you’re on a mission, and you’re about to get caught and interrogated, it’s widely accepted as a really bad idea to have a USB stick containing all the details of your criminal and quasi military activities and all of your personal information on a chain around your neck. That’s why you have to hope that you’re in a Thatcher statue factory when it happens so that you have somewhere suitable to put it. Or alternatively, when the other members of the group suggested even having the USB stick in the first place, as a way of trusting each other, you could have just said ‘No, that’s the stupidest idea I’ve ever heard in my life, let’s just trust each other because we –er – actually trust each other.’ Even better, you could have decided that you didn’t trust them, and agreed to have nothing to do with them because they are quite clearly morons who will very soon be killed. ‘Ajay’ sadly, chose neither of the above, and ended up with a bullet in his head.

And then there’s that reveal. It was the little old lady at the beginning. We didn’t see that coming did we? No, of course we didn’t – even Holmes didn’t – because she’s a tiny, obscure character with no rational motivation. It worked flawlessly for countless seasons of Scooby-Doo – and I half expected her to say ‘Yes, and I would have succeeded if it hadn’t been for the meddling high-functioning sociopath’ – but it won’t wash here. Why did she betray the operatives? She bought a lovely cottage in Wales with her payoff money, that’s why, even though she thought she might end up dead (a lot of deaths in this episode). Instead, being a little old lady, she behaved like all little old ladies do, pulled a gun and killed Mary.

Mary? Bloody glad she’s dead. Maybe now we can get on with Holmes just solving mysteries. Just hope it isn’t another of those fakes. Unfortunately it looks like we’ve got a whole episode coming up where Watson gets over his hatred of Holmes. Yawn.