Who did Line of Duty's Jo Davidson think her dad was – and could he be the key?

Acting DSU Joanne Davidson is languishing in a cell at HMP Brentiss, with scowling warders waiting for the security cameras to mysteriously malfunction so they can duff her up (or worse).

Meanwhile, anti-corruption unit AC-12 is falling apart at the lanyard-clad seams. Supt Ted Hastings is going all wibbly in the glass lift. DI Steve Arnott looks increasingly like a confused teddy bear in a waistcoat. DI Kate Fleming’s cheekbones are getting sharper by the minute with the stress of it all. Frankly, 15m frazzled viewers feel her pain.

After a packed but inconclusive penultimate instalment, next Sunday’s series six finale has more threads to tie up than a shoelace factory after an earthquake. There are fears that showrunner Jed Mercurio might have plotted himself into a corner. If anyone can pull off a narrative escape trick, though, it’s the thriller-writing maestro.

One of the most tantalising and intriguing loose ends is the puzzle of Jo Davidson’s adoptive dad. Among the myriad revelations in episode six was that murder detective Davidson (Kelly Macdonald) didn’t know that “Uncle Tommy” was her biological father. The man she thought was her father was a corrupt police officer. AC-12 is pretty interested in bent coppers, as you might have gleaned. Surely this one is no exception?

There were three mentions of the unidentified crooked cop during this episode. Yet each time, the script stopped agonisingly short of naming him. If we know Mr Mercurio, and we like to think we do, that’s no accident.

First, when Davidson and Fleming went on the run, Thelma & Louise-style - consider them armed and fabulous - Kate (Vicky McClure) demanded answers about Jo’s family. Fleming told her boss that AC-12 had found DNA in her ex-girlfriend Farida’s house which proved she was a blood relative of deceased crime don and all-round wrong'un Tommy Hunter (Brian McCardie).

“My mum was Tommy’s sister,” admitted Davidson. “My dad was… bent, a police officer.” Did that brief hesitation mean she was about to name him but thought better of it? If so, why? Who or what is she so scared of?

When dodgy Davidson was later brought in by AC-12 for that bravura 29-minute interview scene - complete with performative sighs, smirks and eye-rolls from DCS Patricia Carmichael (Anna Maxwell-Martin) - the question came up again.

Davidson was understandably devastated by the queasy news that her uncle was also her father. Yep, those fateful “runs of homozygosity” were mentioned. Cue some powerful acting by Macdonald, all wobbly chin and uncomprehending eyes. “No, no,” she sobbed. “He was my uncle. That…that… that’s not true.”

After copious “no comment”s, it transpired that vile abuser Tommy had raped his 15-year-old sister Samantha and forced her to go through with the pregnancy, controlling her life with coercive bullying. When daughter Jo turned 16, he ordered her to move south from Glasgow and join Central Police, where she could join his network of corrupt officers and do his bidding.

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Tragically, Samantha killed herself in despair at her daughter being at the mercy of “Uncle Tommy” - who always hid from Jo that he was actually her biological father. “So who did you believe was your father?” asked Arnott (Martin Compston). “This person, did he control you the same way Tommy did?”

Before Davidson could respond, this line of inquiry was frustratingly shut down by both Carmichael (for her own nefarious reasons) and Davidson’s Police Federation rep (in a bid to protect his client from further distress).

The parentage riddle popped up for the final time when dream duo Arnott and Fleming were reunited in an underground car park. “Did Jo say anything about her family history?” asked Fleming. “She told me her dad was a police officer.” “No,” replied Arnott. “She never even gave us that much.”

Third time unlucky. When the credits rolled, we were still none the wiser about Dastardly Daddy Davidson. Are seeds being sown for a shock reveal in the grand finale? Who could it be? Might the reason that Davidson is so reticent about both her father and the string-pulling “Fourth Man”, aka “H”, be that they’re one and the same?

If so, the fact that it’s a man clearly rules out any female officers from being the ever-elusive criminal mastermind. Bang go those fan theories about Fleming, Carmichael or DCC Andrea Wise (Elizabeth Rider).

Presumably the fact that Hastings (Adrian Dunbar) was part of the interview panel - and neither he nor Davidson batted an eyelid when the subject came up - means he isn’t the man either. Praise be. We still have our suspicions there’s more to come to light about SuperTed’s early career in Ulster but doubt that a secret adoptive daughter inside the force is one of them.

Who does that leave as possible candidates? Well, two former bent police bigwigs could enter the frame. DCC Mike Dryden (Mark Bonnar) and ACC Derek Hilton (Paul Higgins) not only had Scottish roots like Davidson but theirs were among the case files glimpsed on Arnott’s computer desktop when he ignored his latest drug-test email from Occupational Health. Significant? Don’t rule it out.

Silver fox Dryden resigned from the force in series two after receiving a suspended sentence for perverting the course of justice. Shifty operator Hilton, who appeared in series one and four, was secretly affiliated with organised crime and later found shot through the mouth - likely assassinated by the OCG but staged to look like suicide.

The newly murdered DCI Marcus Thurwell (James Nesbitt) is surely too young, despite that grey dye job, as is PCC Rohan Sindwhani (Ace Bhatti). It could conceivably be a member of the Sands View boys’ home paedophile ring from series three but unless it’s someone we’ve met before, such as CS Patrick Fairbank (George Costigan), that would seem a swizz.

That leaves Chief Constable Philip Osborne (Owen Teale), the high-ranking officer who’s been increasingly looking like the “big bad” of the series, as our prime paternity suspect. Small wonder that Davidson is wary of naming such a powerful figure. Perhaps that’s why Osborne’s pompous speech about “the enemy within suffering the consequences” was intercut with her imprisonment in that closing sequence.

Like Arnott sympathetically told Davidson: “These things we’ve discovered about your family history - nobody can be blamed for what they’re born into or the path it puts them on.”

Who’s the daddy? We’ll have to wait for the must-see finale - but it might just prove key to unlocking Mercurio’s sprawling conspiracy of clandestine corruption. As Hastings put it: “Jesus, Mary, Joseph and the wee donkey.”

Line of Duty continues on Sundays on BBC One. If you’re up to date with the latest series, you can read our Line of Duty episode 5 recap here. Check out the rest of our Drama coverage, or take a look at what else is on this week with our TV guide.