Line of Duty is returning for a sixth thrilling season with Fleming, Arnott and Hastings squaring off against a new adversary – Kelly Macdonald’s DCI Joanne Davidson.

The crime thriller has become Britain’s most addictive and highly anticipated drama series over the last decade thanks to its mix of jaw-dropping ‘Mother of God’ twists, nerve-shredding tension and good old fashioned mysteries (Just who is H?).

Another string to the show’s bow is the incredible guest stars it brings in with every new season. From Lennie James, Jessica Raine and Keeley Hawes to Stephen Graham, Jason Watkins and Thandie Newton, the series uses the big names smartly to keep us guessing – some survive the series while some are killed off in the first episode, and we often don't know which side of the law they’re on until the very end.

Line of Duty season 6: Everything you need to know

Vicky McClure in Line of Duty BBC

Writer Jed Mercurio uses the show’s rich character history to remind viewers of the connecting line across the series – you’ll often see former characters pinned up on a wall of suspects in AC-12’s offices – and occasionally he’ll even bring them back for a cameo or second run.

Ahead of the show’s sixth season, here are five characters we would love to see back in Line of Duty for one last time.

We’ve excluded those who have been killed – RIP Dot Cottan and Lindsay Denton – whose only possible return would be in flashback cameos.

1. Nigel Morton

Neil Morrissey as Nige in Line of Duty BBC

Played by: Neil Morrissey

Last seen: Mowing his lawn in the season 3 finale

Nasty Nige was one of the early villains of the show, appearing in the first three seasons playing the dodgy DC who was up to his neck in shifty business alongside Gates (Lennie James) and Cottan (Craig Parkinson).

At the end of season 3, we saw Nige enter retirement with a hefty pension and disability benefit – something he managed to secure by conning everyone that he had a dicky leg.

Morrissey was brilliant as the snide and sneaky detective and we’re not the only ones keen for him to make a comeback.

Adrian Dunbar (Superintendent Ted Hastings) revealed on the BBC’s Obsessed With podcast that he’s backing the ‘Bring Back Nige’ campaign, largely because Morrissey always took the cast on the best nights out while filming.

2. Detective Chief Superintendent Patricia Carmichael

Anna Maxwell Martin in Line of Duty season 5 BBC

Played by: Anna Maxwell Martin

Last seen: The season 5 finale

She only appeared in the final two episodes of season 5, but Anna Maxwell Martin made a massive impact as DCS Carmichael was dropped in to determine whether Ted Hastings was a wrong 'un.

The leader of AC-3, she was a clinical and cold investigator, who had a bee in her bonnet about proving that Hastings was one of the bent coppers that he likes to talk about all the time.

Even after the dramatic conclusion of season 5, Carmichael still wasn’t convinced that Hastings had acted with total integrity and that he hadn’t revealed John Corbett’s true identity to Lee Banks and got him killed. However, when one of her own team, Sergeant Tina Tranter, turned out to be corrupt, she was forced to leave with her tail between her legs.

We’d love to see Carmichael back at some point – either teaming up with AC-12 to nick some bent coppers or poking her nose in the team's business and causing more trouble.

3. Lisa McQueen

Rochenda Sandwell as Lisa McQueen in Line of Duty BBC

Played by: Rochenda Sandall

Last seen: Season 5 finale - under witness protection for giving info on OCS

Stephen Graham’s performance as John Corbett and his tragic and unforgettable exit unsurprisingly stands out as the most memorable moment of season 5.

But in our opinion, Rochenda Sandall’s Lisa McQueen was the more interesting foe for AC-12 and even as she ended season 5 collaborating with the police in exchange for a new alias, we still couldn’t quite pin down her slippery character.

She may have started offering advice to youngsters on avoiding a life of crime and giving over info on the organised crime syndicate, but she also withheld information on Ryan Pilkington's involvement in Corbett’s death.

The show’s most intriguing character since Lindsay Denton, it would be great to have her back in season 6 in some capacity.

4. Roz Huntley

Thandie Newton as Roz Huntley in Line of Duty BBC

Played by: Thandie Newton

Last seen: Banged up with only one arm

The main guest star in season 4, Thandie Newton didn’t waste any time making a big impact, killing off Tim Ifeild (Jason Watkins) in a terrifying chainsaw scene.

Newton’s increasingly desperate and cunning villain was in a similar mould to season 1 protagonist Tony Gates, but unlike Lennie James’ character Huntley made it out of the series alive.

The last time we saw her, she was being locked up for a 10-year stretch for Ifield’s murder and for perverting the course of justice. But with lots of men in balaclavas in the season 6 trailer, which gave us flashbacks to the big series 4 mystery, who better to make a cameo and offer some assistance to the AC-12 team (from behind bars) than a returning Roz?

5. Gill Biggeloe

Polly Walker as Gill Biggeloe in Line of Duty BBC

Played by: Polly Walker

Last seen: Living in a grim seaside house under a new identity

The third ‘H’ revealed at the end of season 5 was Polly Walker’s deliciously sinister lawyer, who had been causing problems for Hastings and AC-12 since season 3 when she did her best to save Cottan’s bacon.

It came as no surprise when Gill attempted to make a dramatic escape from justice with the now infamous “Urgent Exit Required” text message. However, Fleming and Arnott were onto her tricks and there was no dramatic escape route for Gill, who even ended up with a stab wound from PC Tina Tranter for her troubles.

Gill handed over info about the OCS in an exchange for a new identity at the end of season 5, but it would be a shame to waste an actress as good as Walker and leave Gill all miserable at the seaside when she could be back stirring up trouble for AC-12.

She’s also someone with connections to the higher powers in the force, such as DCC Andrea Wise and PCC Rohan Sindwhani, who are prime candidates to be the fourth and final ‘H’.

Line of Duty returns on Sunday, May 21 at 9pm on BBC One and BBC iPlayer.

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