One of the most addictive series to come out of the BBC is Line of Duty, the police corruption drama that’s about to head into its fifth series.
The programme follows a crime-fighting trio as they expose some of the most bent coppers in the police force - with the perpetrators often the people you would least suspect of fiddling the system.
Added to each series’ particular tale of corruption is the long-running mystery of the identity of 'H', the code name for the overlord of all wrongdoing in the force.
We take a look at what you can expect from series five, and when Line of Duty will be returning to our screens.
Big cast announcement
The BBC has confirmed the first big name star for series 5 with Stephen Graham being revealed as a 'balaclava man', who is a person of interest for AC-12.
Graham is best known for the film Snatch, the HBO drama Boardwalk Empire and starring along Line of Duty's Vicky McClure in the This Is England series.
The first picture of Graham in the show was released on Twitter with Graham lifting up a balaclav - a nod to the central mystery in the fourth series.
First look trailer
The BBC teased the show's return in 2019, releasing the first look trailer after Bodyguard's finale.
It doesn't reveal much about the storyline for the show's returm, but it confirms that the over-arching conspiracy from the previous four series including the identity of 'H' has not been forgotten.
We see footage of Fleming and Arnott back in action and we also get a classic Ted Hastings line as he warns: "AC-12 is being met by an institutional cover up!"
Watch the teaser trailer:
Jed Mercurio updates fans
The show's writer gave viewers a little teaser while promoting the finale of Bodyguard.
Speaking from the set in Northern Ireland, he told This Morning: "We’re actually shooting in a storm here, there are trees down in Belfast and it’s providing quite a challenge.
"What we’re doing in this series is we’re answering some of the questions that were left over from series four and we’re also starting a whole new investigation that will carry through the six episodes.”
When is Line of Duty series 5 on TV?
No sooner has a series of Line of Duty ended than viewers start clamouring for another instalment of the hit drama.
Luckily, the BBC has already confirmed a fifth and a sixth series of the programme, so we’ve known for a while that we'll be catching up with AC-12 again.
Adrian Dunbar, who plays Superintendent Ted Hastings, told the Radio Times: "We'll be shooting the next series at the end of this year, some time in the autumn, so sadly we'll all have to wait until 2019 to find out what happens to us all."
On August 7, the show's creator, Jed Mercurio, tweeted a picture of Dunbar alongside colleagues Vicky McClure and Martin Compston behind a camera, with the caption "One month to go till we start shooting".
Compston, who plays DS Steve Arnott, confirmed that the cast would be back in Belfast in September for the new series.
Fans will be a little frustrated at the time scale as it means that there will be around a two-year gap between series four and five ending, but McClure, who completes AC-12’s crack team as DS Kate Fleming, explained that it was because the show’s creator Jed Mercurio had commitments with a new forthcoming BBC series that will star Line of Duty alumni Keeley Hawes.
Vicky told Digital Spy: "I think it's Jed doing Bodyguard because it's quite a big show to write, so that is all [that's holding things up] I think.”
Who will star in it?
Of course, we can expect our trio of heroes – Hastings, Fleming and Arnott – back on screen to weed out the latest corrupt officers to darken their door.
Viewers will be relieved to see Martin back as Arnott, as for one hairy moment during series four it seemed as though Mercurio might have dropped the ultimate bombshell on the series and killed him off when he was brutally beaten with a baseball bat.
However, Arnott survived - although he has been struggling to fully recover from the attack - and will be back with his colleagues at the AC-12 anti-corruption unit.
Fans may also remember series four including another of the unit’s officers, Maneet Bindra (Maya Sondhi), falling foul of the temptation to turn bad as she began leaking some of the department’s files. It’s expected that her story will be picked up again in the coming episodes.
Mercurio plays his cards close to his chest in terms of what he has planned for Line of Duty, but he did hint that we could see some old faces return to the cast – and he teased the possibility when asked about whether series four’s lead, Thandie Newton, would ever be back as DCI Roz Huntley.
He told TV Times: "It was reasonably conclusive that Roz had accepted her guilt and she was serving her sentence, but we never close the door on any living character. There are always ways in which we'd bring someone back for the right story."
What will happen in the new series?
Is there any use in trying to speculate what might come out of the genius mind of Jed Mercurio in the coming series?
The show’s creator is known for his masterful way of twisting and turning the plot in directions we never could have predicted, and his drama is famous for its complex, wordy scenes in AC-12’s interview rooms where many of the shock developments are revealed.
What we do know is that Arnott will still be trying to cope with his mobility problems after the vicious attack that nearly killed him, and series four saw a wedge driven between the trio when Hastings decided to promote Arnott above Fleming. But will the promotion go ahead?
Maneet, who was pregnant last series, seemed to have been coerced into leaking the unit’s confidential files, so we’re expecting to find out more about why she did it and what the future holds for her.
Remember that dramatic shooting at the end of series four when Hastings took down Balaclava Man, the mysterious murderer who had also attacked Arnott? Well, as we were told at the time, it’s actually Balaclava Men, plural, so we’re hoping to see who else is caught up in the violent gang and how it relates to police corruption.
There have been hints that the next instalment could delve further into Hastings’ past, but Mercurio has also said that viewers shouldn’t necessarily expect it to follow the format of previous series.
He told a Royal Television Society panel: "Season five feels like it's very fresh territory. There are things we haven't done before, characters we haven't seen before, and that I think's part of the construction of the series. We have that architecture which allows us to rejuvenate the format.
"Possibly we've arrived at that accidentally but because it would appear now that we have this situation where the audience is intrigued about what we've got to offer based on who the guest lead is going to be, what other characters there are, what the fundamental premise is, we've been very fortunate that as the series has gone on, people have engaged with every story."
Of course, there’s the continuing saga of the mysterious H’s identity, which is a subject so huge that it deserves a closer look…
Who could H be?
The glue holding each series together and which has had us all guessing for years now is the conundrum of who H is.
It seems that the codenamed corruption boss is ultimately responsible for every dodgy deal that AC-12 has encountered, and the rot could go right to the core of the force if H is as senior as is suspected.
A popular theory was that H was ACC Derek Hilton, who certainly appeared shady but was not exactly a criminal genius. But now that Hilton has been found dead, if H continues to pull the strings then Hastings and co will know for sure that it couldn’t have been him.
Which leads in to another possibility – that Hastings is actually H, which would be a huge shock for the characters and the fans. We’ve seen Adrian’s character make some very compromised decisions and it’s clear that there’s plenty about his past that he’d like to keep secret, but as we discover more about his back story in series five we could find out whether he could be that much of a double dealer.
Mercurio told Digital Spy: "The fact that Hilton is a very good fit for H – and Dot Cottan (one of the show’s corrupt officers) would know that he was H – is something that is consistent with the series as it has gone before.
"But the fact that there could be a suspicion that Hastings is H is something that we want to exploit dramatically going forward."
He also hinted that we could look to an old character from series two, DCS Lester Hargreaves, who popped up again in the final episode of series four, saying: "His name begins with 'H' as well – it was something that we were aware of!"
Line of Duty is due to begin airing series five on BBC One in 2019.
Catch up on old episodes with BT TV via the BBC iPlayer app.
Photos: BBC, Rex