Following last night's broadcast of The Final Problem, the third and final episode of Sherlock's fourth season, one thing has become staggeringly clear - Sherlock, as we know it, is done. 

*Spoiler Warning*

Right from the first episode - The Six Thatchers - it was clear that showrunner / writer Steven Moffat had a hit a brick wall and was now forcing his way through it by any means necessary. The very idea of featuring sharks so heavily in the episode - particularly the finale - leads us to believe that even Moffat himself is aware that he's jumped the shark. Taking on director Rachel Talalay - who directed such cultural gems like Tank Girl and Ghost In The Machine - was a fatal error, as it's clear that she had little in the way of subtlety or understanding of how the series worked. More to the point, there wasn't any push-back on any of Moffat / Gatiss' leanings. Setting the finale of an episode inside an aquarium? The voice-over at the start? Look back on the first and second season and point to a time when any of this would have been acceptable.

Although there were some interesting flourishes - particularly the reveal of AGRA - the writing made absolutely no sense. Mary Watson, a highly-trained assassin doesn't clock that she's got a tracking device on her or knows that she's been followed by her husband and his best friend? Suspension of disbelief - even in Sherlock's world - can only be stretched so far before it snaps. By the end, the idea of a secretary of using a mercenary hit squad for her own end and then carrying a gun around with her just breaks the thing entirely. Mary Watson diving in front of a bullet - and probably breaking the speed of sound in the process - just finished it all off.

The second episode, The Lying Detective, was a far better example of tightly-knit storytelling mixed with visual flights into fancy. Moreover, the idea of its central villain - Toby Jones channeling Jimmy Saville by way of Donald Trump - is fiendishly brilliant. Toby Jones' characterisation is truly disturbing and it's so clear that he's just having a blast, cackling through his crooked teeth and sneering smile. It's a ballsy move, particularly when you consider that Saville was a BBC icon and Sherlock is one of BBC's flagship dramas. As a self-contained story, The Lying Detective works and is by far the most satisfying episode this season. Sherlock, high on drugs and realising that all a serial killer needs is money and power to be successful, is particularly chilling and the Trainspotting-inspired trips into deductive reasoning are interesting as well. Watson's reflections of Mary, however, feel almost like a loose end that needed to be tied up rather than a serious examination of grief. In fact, by the time third episode rolls around, it's all but forgotten - as is Jim Moriarty's involvement in the whole thing.

It's only when we finally meet the central villain of the season - and, it has to be said, the entire series - that it begins to come into focus and we realise what's at stake. The second episode ends on such a tantalising note - what if Sherlock and Mycroft had another sibling who's actually some kind of evil genius? - that going into the third episode, expectations were high. How disappointing then that The Final Problem has the makings of a great episode, but botches the follow-through. Sian Brooke, who plays the secret sister Eurus Holmes, is a revelation and it's particularly witty to see both Mycroft and Sherlock cower in her presence. The writing around her is also excellent, showing that unlike Sherlock and Mycroft who rely on deduction and probability, Eurus actively tips the scale in her favour when she wants something. The eventual reveal of Moriarty's involvement in this season, while not exactly satisfying for fans, does make sense in the larger scheme of things. Moriarty's dead, but Eurus is his revenge. The scene where the two meet for the first time is brilliantly shot and the reflective glass makes for beautiful thematic moments.

It's a shame, however, that the entire episode is effectively set in one location and we're not given a chance to see Eurus in full flight. Instead, we see her toying with her brothers (and Watson, who really is just a loose end at this stage) simply because she can. Using games "with context" to propel the story forward seems like a lazy choice, but the realisation is clear - Moffat and Gatiss have run out of ideas, so instead it needs to be brought into landing with a straightforward plot. Yet, even in the landing, it botches it completely. All Eurus really wanted was a hug from her brother? She set up the entire thing just so she could reach out to her brother? The story ends so hurriedly, so forcefully, that you're pushed onto the credits before you've got time to ask questions. For example, how did Eurus all of a sudden gain super-strength and manage to throw Watson down a well? How did she set up the fake chamber where Sherlock was kept? Was it Eurus on the phone the entire time, mimicking a small child? The analogy works - Eurus' intelligence puts her in the sky, but she can't reach anyone and it'll eventually kill her - but how did Sherlock, Mycroft and Watson not clock that it was her voice mimicking the small child?

The Final Problem encapsulates the problem with the fourth season as a whole - it's nowhere near as smart as it thinks it is, but with enough colours and sparkles, it'll make you think it is. Cumberbatch has run the character through to its final stages, whilst Freeman is manfully serving out his sentence. Although Sian Brooke gave a brilliant performance, she was deployed in the series much too late to make an impact whereas Toby Jones, on the other hand, was given a much neater story to work with. Looking back on Sherlock as a whole, it's clear that the wheels were starting to come off the train by the end of the third season. It didn't necessarily derail here on the fourth, but it's certainly jumped the tracks more than once and it was starting to become obvious that Moffat and Gatiss can't push it on any further.

Maybe that's for the best.