Idris Elba has said he would like to see a John Luther movie one day – possibly an origins story. This would not just befit a character who is more superhero than sleuth, but also seems like the only conceivable way of telling a fresh story.

Although this episode ostensibly concerns the hunt for Twitter-savvy, scumbag-executing vigilante Callum Marwood, it’s also about the enduring bond between DCI Luther (Idris Elba) and DS Ripley (Warren Brown).

It’s manna from heaven for fans of John Luther’s coat in this episode (surely the greatest villain we’ve yet to meet in the show is a killer with a fetish for grey tweed outerwear) as a search of his house by anti-corruption cops Stark and Gray reveals not just one but a whole rack of them.

Luther polarises opinion like a Marmite commercial starring Russell Brand. To some, it’s the only home-grown series that can hold its own against European crime heavyweights like Spiral and The Killing. To others, it’s a car crash of cop clichés; only the herculean efforts of Idris Elba in the title role holding together a show which looks stylishly gritty on the surface but lacks substance where it counts – all woolly grey overcoat and no Kickers.