For those in the former camp, the finale of the last series in 2011 – in which the recalcitrant detective played dice against a killer strapped with explosives in the back of a delivery lorry – seems a lifetime ago. For those who considered this to be Luther/van dross, it probably hasn’t been long enough.

But even if some people only tune in for the premiere of the new series to see if rumours of the Met’s most memorable maverick actually taking his coat off are true, a tenner says they keep watching right till the end. Because this isn’t merely the best episode of Luther yet; it’s addictive viewing of the highest calibre, sodden with claustrophobic tension, punctuated by shocks so heart-stopping that a pacemaker ought to have been given away free with this week’s Radio Times.

The opening titles haven’t even rolled before the first pass-the-beta-blockers moment. After Luther and sidekick Justin Ripley (Warren Brown) bring an anonymous crime git to justice, a woman – Emily Hammond – is murdered in her home by an intruder who unrolls himself from under the bed with the grisly elastic elegance of Eugene Victor Tooms from The X-Files. The killer also dresses the body up in a wig and makeup, gothic stylee. ‘It’s a very specific look,’ Luther remarks. ‘There’s a bit of a Siouxsie and the Banshees thing going on.’ Actually, the corpse looks more like Noel Fielding after six days on the sauce, but there’s no time to ponder such things as he is immediately handed another homicide to investigate: the torture and slaughter of an internet troll named Jarrod Cass.

Luther shows little interest – but this is a mistake, as Ripley discovers when he is summoned to a meeting with former colleague Erin Gray (Nikki Amuka-Bird), now ‘rooting out dirty coppers’ in the anti-corruption unit. Given Gray’s previous run-ins with onetime superior Luther, it’s hardly a surprise to find her former boss is the primary focus of an investigation into serious misconduct. DSU George Stark (David O’Hara), her new senior officer, has even more of a hornet in his helmet about Luther.

‘Do you really think he’s innocent?’ Stark sneers, sounding like Liam Neeson at his ragiest and most bonkers. ‘Do you think he’s some kind of lightning rod who just gets struck again and again?’

Loyal Ripley does, but still ends up wearing a wire as he and Luther visit the Cass crime scene: a high-rise hovel resembling the aftermath of a fight between Banksy and Neil Buchanan. Luther crosses yet another line by dangling a loan shark from a balcony and although Stark is listening in, Gray can’t reach the scene to get visual proof of their target’s ill behaviour. But nor do they see Luther displaying his fundamental compassion, as he does later when trying to console Kev Barnaby (Lucian Masamati) – bereaved parent, victim of Cass’s online abuse, and very obviously the man who slew the troll. It’s a wonderfully self-conscious scene, played to awkward perfection by Elba and Masamati, and an understated highpoint of the episode.

Yet it’s not long before Barnaby is involved in something far more jarring: one of the most strikingly ghastly scenes in the recent history of British TV drama. Luther calls the grieving father and asks him to come in for a routine fingerprinting. It’s anything but routine, of course – and Barnaby knows it. So he goes into the kitchen, turns on a blender, and sticks his hand in. We are spared the grisly details (there are no prosthetic digits getting carved up in a sea of fake blood), but the momentary flash of anguish on Barnaby’s face and the noise of the food processor is as grim as a holiday in an abattoir.

There are more assaults on the nervous system before the credits roll – Ripley and Luther have a scrap in their office and the killer of Emily Hammond pops up in the attic of his next victim, motionless and mannequin-like under a polythene sheet before leaping at the woman’s husband and shoving his head through the bedroom ceiling – but the liquidising of Kev Barnaby’s hand lingers like a payday loan.

Luther should come with a government health warning.

Aired at 9pm on Tuesday 2 July 2013 on BBC One.

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