Line of Duty: Who is Jo Davidson related to? The best fan theories behind the DNA cliffhanger that broke Twitter

Forget trying to work out the ‘Fourth Man’, identify ‘H’, crack who murdered Gail Vella or celebrate the moment Steve and Kate go for a balti again (“Mate”), there is only on question on Line of Duty fans’ lips this week – who is Jo Davidson related to?

The mystery of Davidson’s family background has been alluded to previously in the series with Kelly Macdonald’s ice-cool copper insisting to her jilted lover Farida Jatri that she had no family.

We’ve also seen her smashing wine glasses and angrily screaming at childhood pictures of herself with an older woman, so there are some dark secrets in her home life.

But the significance of Davidson’s family background came into focus last night as AC-12’s search of Farida’s flat brought back DNA evidence that matched Davidson with another person on their database.

The match means Davidson is a blood relative of someone we’ve seen before in the show – but who can it possibly be?

Here are the best fan theories so far…

1. Tommy Hunter

Played by Brian McCardle, Hunter was the rotten crime boss at the heart of the OCG back in season 1, who was killed off at the beginning of season 2 after an explosive ambush on his police convoy – the kick-off for the Lindsay Denton storyline.

As nasty as they come, Hunter’s legacy runs through Line of Duty. He trained Matthew ‘Dot’ Cottan as ‘The Caddy’. He was a paedophile and on Danny Waldron’s series 3 list with Dale Roach and Patrick Fairbank. He was also behind Jackie Laverty’s murder which has repeatedly weaved its way through the show’s six seasons.

He’s also, like Jo, Scottish, which would be handy for continuity.

But is the nationality link just a red herring? Davidson has either ended up in over her head or was manipulated from the start to work for the OCG. But how would that fit in with her being related to Hunter?

If she never knew Hunter, what power would they have over her to work for them?

And if she did know him, surely her link to the OCG would have been spotted when she joined the police.

Her dad or older brother being the head of the OCG is hardly a small detail that could be skimmed over.

2. John Corbett

Oh Stephen Graham, how we miss you. Tragically killed by rotter Ryan in season 5, Graham’s undercover copper who was hellbent on finding ‘H’ has left a shadow over the sixth season.

His widow Steph has ended up in bed with Steve Arnott for a cuddle and is frequently meeting with Ted Hastings, who bunged her a load of cash to help her out.

Connecting John as a sibling to Davidson might explain why Steph has been so heavily involved in season 6.

If we’re joining the storyline dots, it could also be this connection that brings Ted’s dodgy dealings and Steve’s drug addictions into focus as Steph Corbett is the only person who knows about both of these issues.

Fans had already speculated that Corbett and Davidson might be siblings because the woman in the photo in Davidson’s home looked similar to Anna-Marie McGillis.

McGillis was Corbett’s birth mother, who had a close relationship with Ted Hastings in Northern Ireland and was killed for being a police informant.

Does Davidson have the same agenda as Corbett - justice for Anne-Marie?

3. Terry Boyle

It’s a bit of a wild one, but stick with us here.

Terry is one of the few characters who has been part of the show since season 1. We also know that he’s someone who is being manipulated and abused by the OCG.

If Davidson is related to Boyle, it might explain why she’s willing to go so far to keep the OCG on side. She knows that she if doesn’t do as she’s told, her brother might end up paying the price.

Boyle and Davidson might have been introduced to the world of the OCG by their mother – hence Davidson’s angry glass-smashing scene – and then they could have been utilised as a cuckoo and bent copper ever since.

4. Lindsay Denton

We’ve had throwbacks to every other classic Line of Duty character this season, so why not connect Davidson with the show’s true Queen – Keeley Hawes’s DI Denton?

At the moment, this appears to be based on fan desires for Denton to somehow be involved, rather than any connection to the plot and story.

However, it is the sort of shocking revelation that would cause Hastings to mutter ‘Mother of God!’ and take a deep breath.

5. Jackie Laverty

Best known as Tony Gates’ secret lover, a dodgy money launderer and the body that has spent the last six seasons in Terry Boyle’s fridge, Gina McKee’s character always gets mentioned in elaborate Line of Duty theories.

Could Davidson be a younger sibling of Laverty? She could want revenge for her sister, but has ended up going far too deep with the OCG and not know how to get out.

6. DCC Andrea Wise

Just on the basis that it would be nice for Elizabeth Rider to have a bit more to do other than stare down Ted Hastings and throw a spoiler in all AC-12’s plans, it would be great if this was true.

Could Davidson have connections right at the very top of the force? Someone with the power to keep their family connections a secret? And someone who could control and manipulate her into getting involved with the OCG at the start of her career?

7. Danny Waldron

Another Line of Duty classic character, Waldron was played by Daniel Mays in season 3. He grew up in Sands View boys home, where a paedophile ring operated that included police offers and criminal bosses.

Did Waldron have a sister that he didn’t know about? His childhood wasn’t a happy one, so perhaps there might have been a sibling that he rarely saw or didn’t even know existed.

Jed Mercurio may even have left viewers a little clue in this week’s episode, because it was mentioned that Davidson’s home address was Croxford Street. And who also used to live on Croxford Street? Danny Waldron!

8. Kate Fleming

Maybe it wasn’t flirting and chemistry between Vicky McClure and Jo Davidson. Maybe it was sisterly love.

The theory works on the basis that Davidson knows Fleming is her sister, but Fleming isn’t aware. Maybe Davidson brought her into the team to keep an eye on her and protect her.

It sounds bonkers, but what an emotional moment it would be if one of them has to lose their life to save the other in the final episodes!

9. Matthew ‘Dot’ Cottan

It wouldn’t be a Line of Duty list without The Caddy being involved. Is Davidson Matthew Cottan’s sister? Did she get involved with the OCG at the same time that ‘Dot’ was carrying golf clubs for Tommy Hunter?

While it would be a cute connection to the Caddy storyline, it’s hard to make sense of how the Davidson connection could work. They had no reason to cover up their relationship when he was alive, so surely someone would have known about it?

And if she only discovered she had a brother after his death, it would be pretty far-fetched for her to suddenly end up deep in the OCG web as an act of revenge for someone she never even knew.

Watch Line of Duty seasons 1-5 and the latest episodes from season 6 on BBC iPlayer.