Grace Dent on TV: Luther, BBC1

Until recently I have been evangelical about BBC1's Luther. “You MUST watch it,” I've quacked, cupping the faces of acquaintances in bars, in the manner of a woman still maintaining that watching TV is not merely proper, valid employment, but akin to an art form. Trust me, I'm a TV critic. Series two of Luther was a bleak, complex and visceral affair. Twisted pornographers, claw-hammer-wielding terrorists, corrupt police chiefs, mass horror, multiple corpses, Idris Elba being all pant-meltingly handsome in fine tailoring and a faint sheen of righteous perspiration. Glorious, horrific stuff.

Yes, real horror. Horribly horrific horror. The last series of Luther was so genuinely bleak – particularly those scenes where a killer calmly walked through a busy London office-block in broad daylight butchering all in his wake – that I often wondered aloud how BBC1 ever permitted it. This is the BBC for heaven's sake. The BBC, so cowed of late by fear of giving mild offence to some blithering complaining winnet with a Basildon Bond notelet set in Cleethorpes that large swathes of the schedule is back-to-back vintage Flog It. Of course Flog It – barely coherent tight-arses bickering over a price for stained antimacassars in Barnsley – is horrific in its own sense but seems to cause no offence.

This series of Luther, I'm not so in love with. Horrific it is, indeed, but revolving around such familiar, timeworn, spooky notions I can barely be bothered to be frightened. Luther's new nemesis hides under beds waiting for women to come home. Like a less affable Sulley from Monsters Inc. The baddie waits and waits for silly, vulnerable women who are foolish enough to live alone, who despite double- or triple-locking their front doors and using taxis could never prepare for a bogeyman under the bed lurking. He lurks in the cupboards in a shared house of nurses, watching them titter and drink wine, while he stands styled by the BBC wardrobe department to look exactly like a Crimewatch photofit. Standard “sex-offender chic”: black bomber jacket, black knitted hat, jeans, piercing stare. Or he waits and waits in the attics of daft middle-class sorts then jumps out after dark and first kills the foppish husband then lingers over the death of the doe-eyed Boden-clad wife. Attics, wardrobes and underneath beds, he's from the Hans Christian Andersen school of nefarious knobheads. He's the sort of character one might have seen in Charlie Brooker's beautifully observed crime spoof series Touch of Cloth. In fact tons of series three Luther is purely Touch of Cloth. Perhaps it always was but I noticed it less with Steven Mackintosh and Ruth Wilson in the mix. Also, in past series there's been more focus on the daily goings-on of the police station or Luther's flat which are both now missing too, leaving us skulking around council estates, car parks and kebab shops: more The Bill, less Prime Suspect.

After killing, the Hooded Claw plods broodingly and in a heavily attention-seeking manner along CCTV-filled main streets – clutching a holdall of S&M masks and restraints – using his Oyster card on tubes and buses all the way home to his requisite badly-lit, unhygienic flat, cries a bit, pops on the internet to surf some “I Love Serial Killing, Me, I Do” fansites before texting pictures of corpses to his equally creepy chum. Somehow, it still took Luther two episodes to locate him. The VAT office, TV licensing people or traffic wardens would have tracked him down quicker.

But then, Luther has a lot on his mind. In between investigating the death of a naughty internet troll whose death no one cares about – not Luther, not the viewers – our hero is also fending off a pesky Glasgow hardman police investigator who is vexed at the number of suspects and bystanders Luther kills during a typical case. Gosh, DSU Stark is a terrible fusspot. We all know you can't make an omelette without violently killing a number of innocent or not-proven-guilty eggs. This plot line allows me to spend a lot of time looking at Warren Brown, who I love so much I would him watch reading long excerpts from the Xerox 4250 Workcentre photocopier manual, especially if he did the Thai boxing warm-up moves between pages.

Despite all this, Luther has fallen in love. He's been swept off his feet by a floppy-elbowed, toddler-voiced blonde who crashed into his car but still got his phone number despite whiplash. After one date she was mewling needy, possessive stuff like “I don't want to play games! I'm too old for games! You didn't pick up my call? Where were you!?”

Luther, for some reason – one less believable than the serial-killer plot – finds this sort of carry-on very endearing. The only possible explanation for this bleating woman being on the scene after two episodes is that she's part of DSU Stark's plot to frame him. Either that or she's going to be killed by the serial killer just as Luther realises that she's taught him to open up and love. I know this sounds like a terrible storyline cliché, but so is a bogeyman under your bed.