What happened in Line of Duty season 6 episode 4 – and all the questions we need answered

And now – finally – the bombshell has been dropped. Plus, episode four gave us a whole load of other dramatic developments (and a shootout, and multiple murders, and a smattering of balaclava men).

But will Hastings be forced into early retirement? Will AC-12 be disbanded? What will Ryan do next? Here's what we know, and what we don't know, after episode four:

1. Who is DCI Jo Davidson related to?

Some day, I hope to break major news to my boss by sliding a cardboard folder over the desk and waiting for them to turn the page, while I look on gravely. But if I had a secret as juicy as DS Steve Arnott (Compston) apparently does, I don't think I'd be able to contain myself for long enough to deliver the preamble / context dump. I'd just blurt it out.

"As we know, sir, officers' samples are stored on the system to exclude contamination of crime scenes," he told Superintendent Ted Hastings (Adrian Dunbar). "Now, an officer's DNA isn't speculatively examined on a routine basis, and certainly wasn't back when Davidson joined the force. But because the DNA deposits detected at the house weren't assumed to relate solely to a police officer, they were compared against the whole system. The analysis detected partial matches to a nominal whose DNA is stored on other police databases. It's identified over the page."

In response to Ted's I'm-very-shocked face after turning over the page, he added: "As we know, sir, there's only one credible explanation for Jo Davidson's DNA matching this individual. They're blood relatives."

Ted's response? "Mother of God." Naturally.

So... who is this mysterious blood relative of DCI Jo Davidson (Kelly Macdonald)? Clearly, AC-12 is aware of them already, so it must be someone (male or female - Steve didn't specify) who we've met before, or at least someone who we've heard a lot about.

Jo herself told Farida she has no family, but her next of kin is listed on her police record as Samantha Davidson, and we've seen her chuck a wine glass at a framed photo of her with an older woman (her mother?) in a fit of anger. We also know she was born in 1979, and joined the police when she was 20. She trained at Melton Police College, and her training officer was C Stenhurst. Other than that, we're kind of flying blind about her background.

Is she related to Jackie Laverty? Danny Waldron? Tommy Hunter? Ted Hastings? The mind boggles. Craig Parkinson, who played Dot Cottan, has even weighed in on the theories. No wonder it drove Compston wild thinking about it for months – at least we'll (hopefully) get our answer next week when episode five airs. Until then, have your say and vote in the poll below.

2. Did Jo know that AC-12 would make this connection?

We assumed Jo was (rightly) freaking out about the forensics search at the home of PS Farida Jatri (Anneika Rose) because she knew AC-12 would find evidence of her DNA everywhere. That evidence (which they have, indeed, found) exposes Jo as a liar, and backs up Farida's claim that they were in a relationship – and that Jo could have planted the burner phones in her house. Farida's not a fantasist after all.

But maybe Jo was worried on multiple levels! Could she have known that any DNA found in this forensics sweep would be compared against samples on multiple police databases? And that this blood relative would crop up?

3. Why is Jo Davidson working with the OCG?

At this point, it doesn't look like DCI Jo Davidson is the famous and semi-mythical "H" – i.e. the 'fourth man' who is the final remaining pillar of a network of corrupt police officers and organised criminals.

Instead, she seems more similar to season one's guest lead DCI Tony Gates (Lennie James), who was drawn into working with an OCG and then couldn't wriggle out of their clutches, no matter what he did. The OCG's response to Jo attempting to quit working with them (sending messages saying "Job done" and "I'm finished", and then telling Ryan he was being transferred off the Vella case) was to send a big message back to her, by having PC Ryan Pilkington (Gregory Piper) hold a gun to her head and tell her why her resignation has not been accepted.

In case that was too subtle, he also made sure to inform her about Jimmy Lakewell's murder at Blackthorn Prison, for being a "rat" – echoing the time she commented on Farida's firing to Ryan: "That's what happens to a rat."

But why is Jo doing all this? She clearly does not want to be a bent copper. Is she being blackmailed? If so, what has the OCG got on her? Surely this must be connected to her mystery relative?

4. Will Hastings retire? Is this the end for AC-12?

Noooo, not our Ted! The police bigwigs have come up with a dastardly plan to force Ted Hastings into early retirement, while simultaneously hobbling AC-12 by merging it with AC-3 and AC-9 and firing 90 per cent of the lot. It cannot be allowed to happen!

PCC Rohan Sindwhani (Ace Bhatti) and his enforcer ACC Andra Wise (Elizabeth Rider) have long been fed up with Ted, who has this irritating habit of finding corruption within the police force. Ted and his team's sleuthing has the potential to generate negative headlines and bad PR about the police, and Sindwhani can't be having that. Anti-Corruption is a thorn in his side.

So what happens next?

At the moment things aren't looking good for Ted. He was on his final written warning (after the shenanigans of season five), and so far in season six AC-12 has been blundering around making lots of noise but not much headway. Ted doesn't want to be ousted – but if he resists the offer of retirement and full pension, ACC Wise has indicated that disciplinary action lies in his future.

That said, surely Ted isn't actually going anywhere? Surely this isn't the end of the line for AC-12? Jed Mercurio must just be introducing more jeopardy to make the current investigation even more urgent (Ted's got a month, so the clock is ticking).

Plus, it adds further drama to Steve's conundrum over whether to take up the job that's been offered his way; so far, he doesn't know that his current job is even on the line.

5. How did the OCG know to attack the police convoy?

Somehow we've got this far in the article and we haven't even discussed the police convoy shootout! So, let's discuss the police convoy shootout.

Police convoys in Line of Duty are usually ill-fated, and this one (codenamed "Windermere") was no exception.

So, Steve and DC Chloe Bishop (Shalom Brune-Franklin) had a slightly harebrained scheme to grab Jimmy Lakewell (Patrick Baladi) from Blackthorn Prison and whisk him off to a police station, where he could safely snitch on the OCG – away from the prying eyes and ears of corrupt prison guards – and then (hopefully) be entered into witness protection.

But the OCG knew exactly what the plan was, and was well-prepared to make sure that couldn't happen. As the convoy sped through town, two big cars emerged from side streets to trap the vehicles; balaclava men rolled out tire shredders in a well-practiced move and the van tumbled to its side. It was all very pacy and dramatic, and Steve got to show off a bit of heroics, and at the end of the gunfight yet another police officer lost her life. RIP Jonesy.

So, who's the leak?

Presumably Steve's whole plan was designed to avoid prison officers knowing that Jimmy Lakewell was being sprung from jail, so it would be a serious failing on his part if the OCG found out that way.

The other suggestion is that the leak came from within AC-12. Perhaps even Chloe? After all, she swapped places with Jonesy – and then Jonesy was the one who was shot. But it seems more likely that was just down to the chaos of the attack.

Or perhaps, when Steve obtained his "production order" and started enquiring about witness protection for Jimmy Lakewell, suspicions were aroused at a higher level? Perhaps "H" even caught wind?

So far, so mysterious.

6. What did Jimmy Lakewell tell Gail Vella?

Gail Vella seems to have uncovered something – or some things – that incriminated some very powerful people. Most of the audio files and notes she'd gathered as research for her investigative podcast have been "disappeared" (likely nicked in a burglary in the minutes after her death), but somehow the police's tech genius Amanda Yao (Rosa Escoda) recovered a proper clip of Gail's interview with Jimmy Lakewell, which hinted at some intriguing information.

Before the events of this episode, the posh solicitor (now, former solicitor) had been in prison since the end of season four.

As a self-proclaimed coward, he was scared of falling foul of OCG-affiliated prison officers, so he had no desire to speak with AC-12 at any point in this episode. He also had no trust that he'd be safe from reprisals, even in witness protection: "There are people you can't be protected from."

But he did speak with Gail Vella for her podcast, down the phone line from Blackthorn Prison. Even if she reassured him that his voice would be obscured, that must have been a risk (especially with other prisoners and officers potentially listening in). What was he trying to tell her, what did he manage to tell her, and why was it important enough for him to risk his life despite his cowardly nature?

In the clip, Jimmy first discusses the events of season one – when Karim Ali was shot in a counter-terror raid, and all the police officers except Steve agreed to a cover-up. Steve was subsequently transferred to AC-12, and his old boss (the author of the cover-up) is now Chief Constable. Jimmy points out that there was clearly a cover-up, to prevent "reputational damage". He then adds: "But sometimes the reasons for the cover-up are more obscure... well it's got to be something pretty bad if you'd rather go with the story that you're basically a bunch of racists." This baffles Gail, who says she didn't detect a race element and asks about "balaclava man", to which Jimmy responds: "Not balaclava man, balaclava men."

Hm. Unless Amanda can work her magic on any more of the audio clip, that's as much as we're getting, because Jimmy has now been offed in prison by Lee Banks (probably related to the late Carl Banks; played by Alastair Natkiel). The OCG either thought he snitched, or decided not to chance him snitching in future. Plus, it was a good way to send a message to Jo Davidson and to Buckells (Nigel Boyle): "You see what happens to a rat."

Now, the question remains: what did Gail find out before her murder, and can AC-12 continue her work to find out the truth?

7. Is the Chief Constable "H"?

We haven't heard much about Chief Constable Philip Osborne (Owen Teale) since season one, but now he keeps cropping up. Having organised the cover-up of Karim Ali's murder back in the very beginning, we know he's a bad egg; plus, the order to disband AC-12 and get rid of Ted seems to have come from him, via Sindwhani and Wise (who were only too delighted to carry it out).

Osborne is senior enough to be a figure like "H". And it would be quite a Jed Mercurio move, to bring it all full circle in unveiling his final "H".

8. Will AC-12 keep pursuing Buckells?

Oh, Buckells, you fool! Steve Arnott has his doubts about Buckells being a criminal mastermind, and so do we. His level of misbehaviour is more on the level of sexting with female suspects like Deborah. ("Will u show me your trunshon" / "And my helmet"... full body cringe. Ew.)

Buckells claimed the documents about the burglary at Gail Vella's place were planted in his car boot (probably true). He claimed he wasn't the one to secure Ryan's recruitment (possibly true), and that he didn't remember him as that annoying youth offender from Kingsgate Police Station back in 2009-12. He also claimed he didn't call in a favour with Deborah to incriminate Terry.

Under questioning, Buckells had no good answer for why he didn't put in for "intrusive authority" for the surveillance of Beechwood House, but that looks more and more like incompetence than corruption. Buckells is a man who has consistently failed to take responsibility for anything, and who prefers to leave it to his colleagues to do the work and make the decisions. That's the most likely explanation for why he delayed the operation to arrest "Ross Turner" overnight, deciding to seek executive-level sign-off so he didn't have to make the ultimate judgement call.

So will AC-12 continue to go after Buckells? And if they do, what will Buckells say – now that he's been warned about what happens to a rat in prison? Buckells isn't a brave man, or a particularly talented man, and he is now badly spooked.

9. Does Kate really not suspect Jo Davidson?

Last week, we said in our Line of Duty episode 3 recap that we were waiting to see whether DI Kate Fleming (Vicky McClure) would tell Jo Davidson about Ryan Pilkington's true identity – or not. That would give us a good indication of whether Kate actually trusts Jo, or whether she's just trying to make it seem like she does.

And in this episode... she did tell Jo! Despite Ted deciding that Jo didn't need to be issued with an "osman warning" that Ryan was stalking her, Kate made her own decision to tell her boss/friend Jo that Ryan was keeping an eye on her. She then gave her a heads-up about Ryan's OCG past.

Also, in a car-window-to-car-window catch-up with Steve, Kate told her old AC-12 buddy that Buckells was clearly the guilty party and Jo was doing a great job on the Gail Vella case. Perhaps she genuinely believes it?

But now Kate has a new reason to be suspicious, going into episode five. At first, Kate was gratified to see that Jo had taken her warning on board, as she told Ryan that he was being transferred off the MIT and Operation Lighthouse. Clearly, Jo was getting rid of a huge potential threat to the integrity of the team.

But then, after Ryan had paid Jo a visit at home to explain a few realities to her at gunpoint, Jo suddenly reversed course and kept Ryan on the team. Even worse, she suggested that Kate could actually be fired for her own suspicious activities in tailing Ryan's car (just before the attempt on Terry Boyle's life and the murder of PC Lisa Patel). What will Kate make of all that?

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10. What about the guns?

Kate and DS Chris Lomax (Perry Fitzpatrick) are following a new line of inquiry into the murder of Gail Vella – and it's possible they'll discover something that will put them in danger. DCI Jo Davidson certainly seems nervous about what they're up to, and if Ryan finds out he's unlikely to be pleased either.

Gail's murder was carried out using the close contact method, with a workshopped gun and bullet – making it untraceable. But Kate, having made the connection that the armed robbery at the bookie's was likely set up by the same OCG, started wondering about the weapons used in that crime and where they came from.

None of the guns used in the bookie's robbery were workshopped, but Kate pulled in one of the armed robbers (Jake) who revealed that he had, in fact, been offered use of workshopped guns (which he declined). According to his testimony, he was collected from his bed and taken off to an industrial estate – "like a big shed" – where there was a crate of workshopped guns. But the robbers didn't trust the look of them, and declined to use them. So there was a link after all! And a potential location to investigate!

11. What will Ryan do next?

At Kate's request in an underpass meeting with Ted (in which they finally had a proper talk about Kate's decision to leave the team), Ryan has not been pulled in for investigation by AC-12. Kate doesn't want to alert anyone that he's been rumbled. But that does mean that Ryan is still at large – and free to do as he pleases.

Ryan could kill Kate. He could kill Jo. He could still kill Terry. Now that Jo's well aware of Ryan's true identity, and now that he's intimidated her into following orders, he can do whatever he pleases. So – how is he going to play it?

12. Has Steve been found out for his addiction and injury?

If you keep deleting or dismissing urgent emails from the occupational health team after a "routine" drugs test, the problem does not actually go away, as Steve will surely find out in the next episode. But how much trouble is he in? And will this affect his dilemma over whether to leave AC-12 or not?

Line of Duty continues on Sundays at 9pm on BBC One. Take a look at the rest of our Drama coverage, or check out our TV guide to see what's on TV this week.