Grey coat, red tie. Artfully furrowed brow. A gruff, East End voice, administering exclamations of doubt (“It’s not right, is it?!”) and fury. John Luther (or Loofah, to be precise) is a detective, but he is also a deeply troubled man with a brilliant mind, whom creator Neil Cross has described as having “some of the Sherlock Holmes about him”. And, when he arrived on our screens some nine years ago – played by Idris “Future Bond” Elba – it seemed that Cross had cracked the formula for a compelling British crime drama, darting from gangland gore to comic-book camp.

In the show’s first series, a crucial bond was forged between this not-bent-but-also-not-entirely-unbent copper on the brink of self-destruction, and child prodigy-turned-killer Alice Morgan (Ruth Wilson). In the foreground were a series of heinous cases involving snipers, kidnappers and killers but, ultimately, the biggest threat turned out to be one of Luther’s own colleagues, DCI Ian Reed, a bent – yes-actually-properly-bent-this-time – cop. Series two offered more personal and professional strife, the antihero flanked by his more rule-observant sidekick, DS Ripley, and the self-righteous DS Gray. Sure, every baddie ultimately fitted the mould of “middle-aged man surviving on Rustlers burgers in his mum’s basement”. But when it worked, Luther really worked, making up for its pick-and-mix approach to thriller tropes with a brooding core, as echoed in its brutalist backdrops. (“This London – soulless, anonymous, thoughtlessly modern – is the very image of a 21st-century abyss,” mused the New York Times in 2011.)

And yet, Luther couldn’t quite sustain its bathos. The moment things started to go off the boil is open to interpretation. Was it series three, with plummy antagonist Tom Marwood, who looked as though he was mildly irked by an Ocado substitution as he sought retribution for his wife’s murder? Was it the needless death of poor young DS Ripley? Or was it series four, where Luther took on the hackneyed role of a cop coming out of retirement for a grand total of two episodes? (Elba and Cross were both pressed for time). By the time series five debuted at the beginning of this year, it wasn’t so much a case of “It’s not right, is it?” as “Something’s gone a bit wrong”. Between the dated, Kray-lite schemes of gangster George Cornelius, Alice’s personality transplant (the much-missed Wilson had seemingly mislaid her character’s accent since starring in The Affair) and a lazy subplot about the killer having a brain tumour, things felt at best uneven and, at worst, entirely cringey, with psychosexual themes proving little more than a glossy patina.

Elba recently revealed that a film version of the series is in the works. Translating the show’s Gotham vibes to the big screen might have worked back in 2010, but that was before Killing Eve’s queer cleverness, or even the moral message of Bodyguard. Unless Luther can channel the grit that once made it so gripping, perhaps it’s time for our man to hang that coat up for good – or at least take it for a nice dry clean.