Line of Duty: Five unanswered questions for the series six finale

The finale of Line of Duty's sixth season is likely to have big implications for the AC-12 anti-corruption unit

Jesus, Mary, Joseph and the wee donkey - the latest series of Line of Duty reaches its dramatic conclusion later.

The penultimate episode of series six of Jed Mercurio's long-running police corruption drama was the show's most-watched to date.

Eleven million UK viewers tuned in last Sunday to learn secrets about Acting Detective Superintendent Joanne Davidson's family background - that's more than the 9.85 million who watched The Oscars in the US on the same night.

Jo Davidson's family history is under scrutiny

The record ratings came as a trailer for the finale suggested that it may, for better or worse, bring an end to AC-12's hunt for bent coppers.

"Every investigation has led to this," it teased.

But as the puzzle over Gail Vella's murder - and the identity of elusive organised crime kingpin H - appears nail-bitingly close to being solved, the evidence remains far from clear.

Here are five questions that need to be answered before the case can be closed.

1. The identity of H

It's the question that's haunted AC-12 more than any other - just who is the mastermind behind the covert network of police officers colluding with the organised crime group (OCG)?

Beyond their modus operandi of giving orders via encrypted laptop messages, we know nothing concrete of their identity.

The hunt began with corrupt officer Matthew "Dot" Cottan. In his dying declaration at the end of series three, he appeared to blink the letter H to signal the name of the person in charge.

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The intervening years have uncovered police chief Derek Hilton and the force's legal counsel Gill Biggeloe as corrupt senior figures within the OCG - but neither spearheaded the operation.

This left AC-12's Kate Fleming and Steve Arnott to re-examine the evidence by the end of series five. Had Dot, in fact, been tapping his finger four times to communicate the number four - suggesting a final shadowy figure is yet to be unmasked?

Here are the main contenders of the fourth man (or woman):

Philip Osborne

Philip Osborne holds immense power, but does he put it to best use?

Now chief constable of Central Police, Osborne is the prime suspect, according to a recent YouGov viewer poll.

On present evidence, this makes sense. Right from the first season, when acting as head of anti-terrorism, he tried to pressure Arnott and his team to cover up the death of an innocent civilian.

Still, this dubious behaviour didn't stop his promotion to top office - much to the disgust of AC-12's boss Ted Hastings - and season six has only added to the questions over his integrity.

On a personal level, it's been revealed he was one of the officers led by Marcus Thurwell (another key suspect) on the case of Lawrence Christopher, a young black man killed in a racially-motivated attack and failed by the police.

This matters because Gail Vella had been exploring links between the police and the OCG before she was murdered, with the Christopher case seemingly high on her agenda.

Chief Constable Osborne has also used his current powers to seemingly obstruct and weaken AC-12's investigations.

Not only has he sanctioned the decision to merge and cut the unit, he's also the superior of Deputy Chief Constable Andrea Wise, the thorn in Hastings' side who cornered him to retire, and has sent in Detective Chief Superintendent Patricia Carmichael to take over anti-corruption operations.

We also know Osborne was behind the call to pull surveillance on Jo Davidson and Ryan Pilkington (both in clutches of the OCG) on the night they attempted to lure Kate Fleming to her death.

Marcus Thurwell

James Nesbitt is yet to appear in character as the elusive Marcus Thurwell

A new character in season six, Thurwell's influence on events, past and present, seems far-reaching.

As the senior investigating officer in the Christopher case, its failings fall on his shoulders. This includes appearing to protect suspects in the case, namely the son of OCG boss Tommy Hunter.

He's also connected to the Sands View Boys Home abuse scandal.

But after apparently being found dead during a police raid on his Spanish retirement property in last week's episode, his value as a suspect will depend on whether further messages are sent...

Ted Hastings

Is Ted Hastings as honest as he seems?

Mother of God! Could the man seemingly most committed to rooting out bent coppers be the worst of them all?

Despite being cleared of much wrongdoing after being framed by Gill Biggeloe, questions remain over his private prison trip to meet OCG honcho Lee Banks in the last series.

A few weeks back, Steve discovered, while visiting Banks himself to question him about Gail Vella, and his brother Carl Banks' possible involvement in her death, that Ted had told him of a "rat" in their gang.

His name? John Corbett, an undercover officer embedded deep within the OCG.

Read on to learn about the mystery surrounding the dirty 'savings' money he gave to Corbett's widow, and the potential importance of his poor spelling.

Patricia Carmichael

Smirk or smile? Anna Maxwell Martin plays Patricia Carmichael

AC-12's nemesis Carmichael openly distrusts Hastings - building a case against him last season based on Biggeloe's planted evidence that ultimately fell apart.

A cold, perennially poker-faced figure, forever willing to do the bidding of her boss Osborne, it's unclear how far her influence goes as she sets about dismantling the unit.

2. The mystery Corbett money

Steve has discovered Ted's secret 'dirty' money donation

Viewers already know John Corbett's widow Steph and their children are living off £50,000 of dirty OCG cash gifted by Hastings (who had received it as part of an attempted OCG bribe) in the last series.

But this season, Steve discovered the (literal) cash in the attic during a not-so-legal search of her home, after spending the night getting very close to his late colleague's wife.

When confronted, Steph claimed to have invested "John's life insurance" money in a friend's hairdressing firm. Committed fans have since remembered that Steve had suspicions over a hairdressers in series one, and was ultimately proven correct when it turned out that criminal real estate owner Jackie Laverty (later brutally killed and dumped in a freezer) had been using it as a money laundering front for the OCG.  

Could they be one and the same, with Steph - unwittingly or otherwise - developing links with the OCG while her husband was undercover?

3. Marcus Thurwell - alive and hiding in plain sight?

Do Marcus Thurwell's eyebrows suggest he may still be alive?

If a character is shown dead on screen, they must be dead, right?

Well, perhaps not in this case. The AC-12 team discovered Thurwell's death in real time, thanks to a live feed provided by Spanish special forces.

However, the footage identifying the body was not particularly clear. Instead, Ted and co took the word of a Spanish officer whose face was largely covered - aside from distinct bushy eyebrows.

Fans have since speculated that these match those of Thurwell, played by Cold Feet's James Nesbitt. Could it be that Mercurio has been saving his star name's on-screen appearance for a shocking finale?

4. Steve's forever-delayed drugs test

Steve Arnott has turned to painkillers to deal with a persistent back injury

This season has proved that Steve, Line of Duty's action hero, is not as indestructible as we thought.

Despite making it back onto his feet after being pushed down a flight of stairs during series four, the injuries - which briefly left him using a wheelchair - appear to have done lasting damage to his back.

Fearing he'll be discharged from field work if he reveals the truth (having just been promoted by Ted), Steve has coped with the back spasms by relying on painkillers. And now he can't stop.

Steve's back pain appeared to vanish during an on-foot chase sequence

After managing to keep his addiction battle under wraps, a flare-up during a visit to Steph Corbett exposed the truth, and she in turn told Ted out of concern.

Now, having determinedly ignored a mandatory drugs test for weeks, he has five days to check in with HR - or risk receiving a yellow notice and being suspended from duty.

A potential perfect storm with Carmichael is looming...

5. Definately guilty?

A stray letter, as featured written in the trailer for series six's concluding episode, may provide a big clue

The perfect crime leaves no trace but, despite contacting turncoat officers through encrypted communication, H consistently misspells definitely with an A. (If only they had used autocorrect!)

Why does this matter, you ask. This isn't a game of scrabble! No, but a certain AC-12 figure has been caught spelling it this way. One Ted Hastings. A man who, last season, went to great lengths to dispose of a personal laptop.

A red herring? The perfect set-up to finally successfully frame anti-corruption's white knight? Either way, a brief reference was made in the trailer for Sunday's finale.

Oh, and to answer the question of whether this really is the last season - no comment.