Your Line of Duty series finale questions – answered

No. It may have been made to look like the Assistant Chief Constable had shot himself but we can be pretty sure that he didn't. Why? Well, let's examine the evidence: Hilton was almost certainly the corrupt 'H' embedded within the top levels of the police force – but he was working for someone who threatened to plant his DNA on a corpse if he didn't comply. "You think Hilton's top dog, how come he bricks it every time a new body's found?" Jimmy Lakewell asked Steve Arnott.

Ted Hastings may suggest Hilton's death was suicide – "he knew the game was up" – but it seems more than likely that the police boss tried and failed to make a run for it. After all, he was found in the same place as poor social worker Oliver Stephens Lloyd (below) who tried to blow the whistle on the paedophile ring uncovered in series three and found himself dumped, dead, in a river for his efforts.

2. So, Hilton was the mysterious 'H'?

It looks like it. The end of the series saw a board erected within AC-12 with all the names of the officers who could have been "H" – the copper named in Matthew "Dot" Cottan's dying declaration as the corrupt senior figure planted deep within the force. We didn't recognise many – among them were murder squad's Lester Hargreaves (who made a comeback in the series finale), Roz Huntley (who turned bad but was ultimately played by the criminal ring) and Ted Hastings (who we can't even begin to consider) – but Hilton is surely the man? He showed his hand when he made a run for it. Can AC-12 rule out the other candidates? Or will series five see them investigate the likes of Hill, Haleton and Harris?

3. How was Roz's wound connected to Tim Ifield?

Tim was a carrier of MRSA. You can be a carrier of the bacteria without showing any symptoms – it lives up your nose or on your skin – but you have the ability to pass it on to others. Now, the police had fibres from Tim's nose thanks to their early suspicions that he might have been balaclava man – DNA which, when tested, proved that he was indeed carrying around MRSA.

Crucially, when he knew he was dying, Tim clawed at Roz and the wound he inflicted became infected with the bacteria. So, when it came to taking a swab from her injured arm, the hospital found the gash to be teaming with the superbug – a connection that linked her with her colleague's murder.

4. How did AC-12 know where Roz had buried the evidence?

Now, this is where things get complicated. Ted, Steve and Kate knew that Roz had left her phone at home on the night of Tim's murder in order to go off grid – and they also knew that she must have had to dispose of her bloody clothes at a later date. As a senior investigator, Roz was wise to every trick in the book, including switching her phone off so her movements couldn't be tracked. It's what she did a few days after Tim's death when she drove to dispose of the clothing, Tim's gruesome amputated fingers and more.

So, with a small window of time to focus on, AC-12 scanned CCTV footage to trace the movements of her car. But crafty Roz knew what she was doing, dumping the items in an area with no cameras and a huge radius. AC-12 were stumped. That is, until Steve had the brainwave that the time she'd given herself to dispose of the incriminating evidence was so tiny that she must have known exactly where she was going. She must have used a site she was familiar with – a site she'd already investigated.

When the team started searching the location of a two-year-old investigation into a domestic murder that she had led, they found what they were looking for. And Roz had run out of places to hide.

5. Why did Roz Huntley confess?

She didn't have much choice, did she? With an overwhelming weight of evidence against her, Roz held her hands up and admitted to AC-12 that she had accidentally killed Tim Ifield. Hastings & co had been right all along and the DNA, murder weapons and clothing they had accrued were in the end too much for Roz to wriggle free from.

But there was another reason why she let her account of that night come tumbling out. You'll have noticed her confession came at the exact moment Jimmy Lakewell was about to excuse himself from the interview citing a conflict of interests. She had to keep him in the room in order to hold him to account for his crimes.

6. How did Roz prove Jimmy Lakewell and ACC Hilton's crimes?

We've not had much time for Jodie this series but at least she turned out to be useful in the end. When Roz asked her to investigate some telecoms activity on the day Steve was attacked, she came up with the goods and we saw the DCI examining the records in the moments before her arrest.

Fast-forward to the interview room and – once she'd confessed – Roz proceeded to show up AC-12 again by cracking their entire case. The calls she was tracking came from three burner phones. The first matched Jimmy's movements and was used moments after Nick Huntley, Roz's husband, called the lawyer alerting him to the fact that Steve was sniffing around the building. The third aligned with the movements of balaclava man and was tracked to the site of Steve's 'fall' minutes later. But there was one more burner phone that connected the two – a number that Jimmy called which in turn dialled balaclava man.

The phone belonged to Hilton who had been foolish – or foolhardy – enough to give the number to Roz in his attempts to sleep with her. The napkin he'd written it on served as a crucial piece of evidence in identifying Hilton as "H" – even if AC-12 failed to get to him before his criminal bosses did.

7. Was Roz trying to frame Michael Farmer?

According to the woman herself, no – and we're inclined to believe her. Roz claimed that Jimmy Lakewell was part of the big cover up but she wasn't. He had represented Michael in the past and knew he'd make for a believable culprit. He also knew that Roz was under pressure from above to crack Operation Trapdoor and that she would willingly accept a suspect – even one with reasonable doubt. It appears Roz was a mere cog in the wheel – not the 'big bad' orchestrating Farmer's wrongful imprisonment.

8. Who was Jamie Desford working for?

Jamie isn't the brightest tool in the box. He fluffed the AC-12 interviews, got blamed for Maneet's covert downloads and almost became Hilton's collateral damage. It's unclear at what point the senior officer recruited his obedient underling but it was definitely Jamie who tipped off his boss that he'd been implicated by Roz. But the young DC had no idea quite what he'd signed up for. Adamant that Hilton had his back, he attempted to herd Jimmy away from AC-12 – blissfully unaware that by doing so he was signing his own death warrant. In the end it was Steve who persuaded him to stay put and hand over the gun he'd been pointing in the faces of all our favourite AC-12 officers.

And we breathed out a deep sigh of relief...

9. Who is DS Sam Railston?

Recognise this familiar face? After Hilton had prevented AC-12 from furthering their investigation into Tim Ifield's death, murder squad were called in and with them DS Sam Railston. Sam was, of course, Steve's girlfriend at the start of series three – we saw the pair living together, looking all lovey dovey as they celebrated their one-year anniversary. But their happiness was short-lived as Dot made an impressive attempt to frame Steve as the Caddy – the corrupt policeman doing the bidding of this mysterious criminal network.

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Steve was even arrested prompting Sam to put the brakes on their relationship. By the time he was cleared, there was no going back and it looks like the two haven't crossed paths since. That is until Steve played on her guilt to get access to Nick Huntley's interview tapes.

10. Who is balaclava man?

Make that balaclava men. According to Jimmy, there is more than one henchman running around with knitwear on his head. One in particular committed Hana's abduction, Steve's assault and planted evidence in Michael Farmer's house, but there are more doing the bidding of this mysterious criminal network. And they have a hold on anyone refusing to cooperate – as Jimmy explained, don't do what they ask and they produce a body from cold storage with your DNA all over it.

Tracking down just who "they" are will no doubt be the focus of series five...