DI Kate Fleming has had her moments, notably that high-speed car chase and winning the gunpoint stand-off with bad lad Ryan Pilkington. So has waistcoated warrior DI Steve Arnott; shooting a sniper while lying on his back in a flipped van wasn’t too shabby. Superintendent Ted Hastings’ highlights, however, have mainly involved smoothing down his lustrous hair or muttering “Mother of God”.

Our AC-12 favourite? It might just be the Line of Duty anti-corruption unit’s newest recruit. DC Chloe Bishop arrived as a fresh-faced replacement for Fleming, who has departed the dream team for policing pastures new. Actress Shalom Brune-Franklin hasn't just made the role her own but become the hit drama’s cult heroine.

As a junior officer (“wee Chloe” to old warhorse Hastings), she was initially assigned to low-level cases. Coppers fiddling their expenses claims or taking too many sickies, that was Bishop’s beat. Yet as AC-12’s investigation has become more hard-hitting, wide-ranging and fiendishly labyrinthine – taking in hired hits, historic child sex abuse and institutionalised racism – Chloe has raised her game like a sort of law enforcement Phil Foden.

With Arnott addicted to popping painkillers and Hastings being ushered into enforced retirement, the unflappable newbie often seems to be the only thing stopping the unit from falling apart. She's AC-12's MVP.

She’s handled her high-octane action scenes superbly, staying cool even when the AC-12 convoy was surrounded by organised crime’s trademark black Range Rovers and bullets started flying. She desperately dragged colleague Sgt Ruby Jones to safety after she was shot by a sniper and administered CPR right there in the road. Her tears when “Jonesy” died injected emotion into an adrenaline-pumping set piece.

When it comes to her everyday duties, Chloe is diligent, resourceful and tenacious. Indeed, it sometimes seems like she’s the only member of the department doing any actual detective work. As one Twitter wag put it: “Breaking news: DC Chloe Bishop rushed to hospital with back problems after carrying the whole case.” Another imagined the following exchange: "Chloe, can you find out everything about all things, then tell me?” “Sure, Guv."

line of duty season 6
BBC/World Productions/Steffan Hill

She’s forever finding crucial CCTV footage or financial records, assembling dossiers or swiping at iPads. It was Chloe who uncovered that murdered journalist Gail Vella was investigating both the Sands View Boys’ Home abuse scandal and the preventable death in police custody of race-hate victim Laurence Christopher.

In the gut-punch scene where Chloe briefed her colleagues on the latter travesty (clearly inspired by the real-life Stephen Lawrence and Christopher Alder cases), Brune-Franklin played the scene beautifully. All focused passion and righteous outrage, she simmered with fury as she recounted the shameful behaviour of bigoted police.

When her boss Arnott said “Sorry you had to dredge all that up” and asked if she was OK, Chloe simply shook her head and replied: “How could anyone be OK?”

Before BBC One’s flagship drama came calling, Brune-Franklin was best known for playing Private Maisie Richards in middling BBC squaddie drama Our Girl, sorceress Morgana in Netflix fantasy flop Cursed and Hugh Laurie’s long-lost lovechild in political thriller Roadkill.

Endearingly, it took Brune-Franklin a while to realise what she was getting into. “I remember receiving an email that said ‘LOD6’ and I thought, ‘Oh, it’s an audition for a show called ‘Lod’,” she has laughed. “I was so focused on reading the scenes, it didn’t click in my head that it was Line of Duty. When it dawned on me, I freaked out.”

Off-screen, St Albans-born Brune-Franklin lives a glamorously Instagrammable life in sun-drenched Western Australia, where her working-class parents emigrated in her teens. On-screen, she dresses down in sober office-wear: plain knits, smart shirts, blazer-and-trousers combos. The 26-year-old’s performance has been unflashy but authentic. She’s an utterly realistic presence. Every office team has a quietly efficient presence like Chloe who keeps the show on the road. Brune-Franklin is fully believable as that lynchpin.

She handles writer Jed Mercurio’s jargon-packed script, stuffed with acronyms and police procedural shorthand, with aplomb. In last weekend’s penultimate episode, Brune-Franklin had to speak lines like: “Alpha Charlie 5-2 to control, active message. State 6 at Menton Road, one police major trauma, request urgent paramedic attendance and full forensic deployment.”

line of duty season 6
BBC/World Productions

Such dialogue is devilishly difficult to learn, let alone speak with a straight face, but she makes it look effortless. From CHIS to NABIS, from OCG to UCO, Chloe speaks the lingo as fluently as co-stars who’ve been doing it for a decade.

Line of Duty is renowned for its epic interrogation scenes (Brune-Franklin admits to ”freaking out” and “fan-girling” when she first entered that familiar “glass box” interview room) but she slotted in like she’d always been there. For the benefit of the DIR, she’s shone.

All manner of fan conspiracy theories have sprung up around the Chloë character. Is she corrupt? Is she leaking police intel to the criminal fraternity? There's been most widespread speculation on social media that she’s secretly the daughter of DCI Tony Gates (Lennie James) from Line Of Duty’s debut series. After all, Gates had two daughters, one of whom happened to be named Chloe, who’d be around Bishop’s age now.

Could she be a female equivalent of PC Ryan Pilkington, who was a gobby kid in series one but grew up to be a gangster embedded in the police force? It’s not impossible but it's unlikely. If she was playing a long game by covertly infiltrating AC-12, why only change her surname, not her tell-tale first name?

Both the Bishop character and her portrayal by Brune-Franklin have been among the most impressive aspects of a phenomenally successful series. Line of Duty’s live viewing figures have nudged 10m, climbing to a record-breaking 15m with catch-up. If there’s another AC-12 case to come – and BBC bigwigs will surely be begging Mercurio on bended knees – Brune-Franklin has earned an even more central role. Plus probably a promotion to DS.

She’s already lasted five episodes longer than Steve’s previous Kate substitute. Series two’s sidekick DC Georgia Trotman (Jessica Raine) survived just a single episode before being unceremoniously thrown from a hospital window to her death. Frankly, it’s a surprise that supercop Chloe wasn’t there to catch her.

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