There is, of course, the most elementary of gags begging to be made here: that when Sherlock Holmes leapt from a hospital roof and splattered on to the pavement below, only to somehow survive unscathed, he spectacularly hurdled the shark mid-jump. But while Sherlock never did hit the same heights post-fall, it’s not for the obvious, death-defying reason.

At first, Sherlock was nigh-on perfect. Steven Moffat and Mark Gatiss took Arthur Conan Doyle’s beloved detective from Victorian London to the busy blur of the modern city, with Holmes reimagined as a “high-functioning sociopath” and recovering drug addict. Conan Doyle’s mysteries were given ingenious 21st-century twists, and Holmes’s remarkable deductions were conveyed not merely via wordy monologues but with stylish visual tricks. It was witty and brainy and gripping, and stuffed with compelling characters – from Holmes’s sidekick John Watson, to his nemesis Jim Moriarty, a deliciously camp crime lord with a fondness for the Bee Gees. It closed its second season with a doozy of a cliffhanger, as Sherlock faked his own death and set up the impossible mystery: how did he do it?

When he returned, his elaborate explanation was picked apart by a sceptical acquaintance, the writers’ way, surely, of shrugging off a potential backlash and gently poking fun at any naysayers. The show had another problem, though. Something had changed, and it began to display the same flaws as its titular sleuth: it became so obnoxiously in love with its own smarts that it lost sight of what made it so brilliant. Where once the stories had been fiendishly clever, now they were needlessly convoluted: episodes were loaded with showy plot twists, narrative rug-pulls and gotcha-reveals that felt like shock for shock’s sake. The tone was increasingly smug, all in-jokes and fan service, including a whole sodding episode set in Sherlock’s “mind palace”, a once-great idea turned into a meta back-slap. Most damningly of all, the nuts-and-bolts detective work got punted into the background to make room for all the other gubbins. The first two episodes of season three spent most of their running time on farce, pranks and hijinks, including Sherlock disguising himself as a waiter before revealing his return to Watson, and the two of them boozing on a stag night, like Baker Street Men Behaving Badly.

It felt less a detective show and more a show about a detective, with much of its focus centred on Sherlock’s growing compassion. But that didn’t quite add up, either: Sherlock was regularly scolded for his narcissism by people who endlessly fawned before him. Registrar Molly Hooper mooned over him despite his cruelty; Inspector Lestrade hero-worshipped him; dominatrix Irene Adler fell in love with him, even while identifying as a lesbian. It was little surprise when the last episode found his long-lost evil sister, Eurus, constructing an elaborate murder trap all because he wouldn’t play with her as a child. Little did she know that she wasn’t missing out: even if the game was still afoot, it wasn’t much fun any more.