Since it began on BBC Two in 2012, Line of Duty has gained a cult following thanks to its thrilling plots, killer twists and complex, relatable characters.

In season 5, the show’s creator and writer Jed Mercurio has rewarded the show's loyal fans by slipping in hidden details that only die-hard viewers would pick up on.

We reveal the Easter eggs you might have missed in Line of Duty season 5, or jump to a specific section from the list below.

***SPOILERS for all series of Line of Duty ahead***

The character comebacks

Line of Duty Ryan BBC One

Vicky McClure (This Is England), who has played DI Kate Fleming in seasons 1-5, told BT TV and journliasts that if you look hard enough, you’ll spot little Easter eggs from previous seasons that tie it all altogether.

She said: “If you’ve watched from series 1, or you’ve gone back and watched from the start, if you look hard enough you’ll find a lot more in this series I think, of returns of things and people.

“This is what Jed [Mercurio] does brilliantly - the unexpected. It always seems to creep up on you when you least expect it, and that’s definitely throughout the whole series, so keep your eyes peeled.”

Keen-eyed Line of Duty devotees may have spotted a few familiar faces making a return to the show over the course of the latest series.

In season 5 episode 1, we were introduced to the unmasked balaclava gang, including John Corbett (Stephen Graham), Lisa McQueen (Rochenda Sandall), and teenager Ryan Pilkington (Gregory Piper), above right, who *spoiler alert* later ruthlessly slashed the neck of undercover police officer Corbett.

Recognise Ryan? He’s no newbie to the murky world of undercover dealings, and has been in the balaclava gang since he was 13, above left.

In series 1, Ryan was an envoy between Tommy Hunter (Brian McCardie) and DCI Tony Gates (Lennie James), transporting burner phones between them on his BMX.

Yep, he’s the lovely young chap who put DS Steve Arnott (Martin Compston)’s fingers in a vice, and repeatedly shouted "Ya bent bastard!" at Gates.

Ryan’s long-standing dealings with, and knowledge of, the police allows him to have an additional insight into AC-12, like him knowing that Arnott wasn’t corrupt, for example.

He’s not the only member of the OCG to make a surprise comeback this season. Gang member Miroslav Minkowicz (played by Tomi May) is another long-standing and loyal member of the OCG.

Like Ryan, he first appeared in series 1 and has history with Steve Arnott, who he beat up when he was working for Tommy Hunter.

While Miroslav was killed by armed police in series 5 episode 5, Ryan’s future in the OCG looks more uncertain. Some fans have predicted that the interview he mentioned to McQueen in series 5 episode 5 was for the police.

Other characters from previous seasons who’ve been mentioned include DI Lindsay Denton (Keeley Hawes), who seems to be haunting Superintendent Ted Hastings (Adrian Dunbar) from the grave.

Die-hard fans will know that Denton, who was referenced in Hastings’ recent interrogation with Detective Chief Superintendent Patricia Carmichael (Anna Maxwell Martin), also previously investigated Hastings’ finances, which got him in all sorts of trouble.

And what about the Caddy? Jed Mercurio tweeted a picture of Craig Parkinson during filming for series 5, and Martin Compston got fans very excited when he shared a photo of him alongside Parkinson recently.

But both Adrian Dunbar and Martin Compston made it pretty clear to BT TV that it’s unlikely DI Dot Cottan aka the Caddy - who was shot dead at the end of season 3 - will make a comeback.

Of character comebacks in series 5, Dunbar said: “I think there might be a couple of surprises, but maybe not the Caddy."

Compston added: “Yeah I think the machine gun to the chest was pretty final. With Jed writing, nothing’s ever as it seems, so there’s a few surprises.”

The secret storylines

Line of Duty jetty BBC One

As well as the character comebacks, there are certain plot points and secret storylines that only viewers of the previous seasons would spot.

First up, you might recognise the jetty where PC Maneet Bindra was left for dead by the OCG at the end of series 5 episode 1.

It’s the same place where social worker Oliver Stephens-Lloyd was found dead (made to look like suicide), as was Assistant Chief Constable Derek Hilton (also made to look like suicide) at the end of season 4, above.

Suspicious? You bet.

Then there’s the whole Jackie Laverty (Gina McKee) storyline. You’ll know that Laverty’s body was found in the body bag at the car pound in season 5 episode 4 (we were made to believe it was Corbett’s before it was revealed to be Laverty).

But who is she, and what does she have to do with the OCG?

Way back in series 1, Laverty was one of the show's main characters, and was revealed to be having an affair with Tony Gates. She was killed off by the OCG and her body was placed in a freezer.

It was revealed in series 5 that parts of her corpse are still in the freezer of Terry Boyle (Tommy Jessop), a man with Down’s syndrome who lives on his own and is used and manipulated by the OCG (another character who's made a return from series 1).

And then there’s the whole Hastings' dodgy payment storyline. Hastings received a huge payment from dodgy lawyer Mark Moffatt as 'compensation' for his ill-advised investment into the Kettle Bell Complex.

Moffat was reintroduced into Line of Duty this series after acting as Roz Huntley (Thandie Newton)’s Police Federation rep in series 4.

There’s also the subtle plot around Hastings and Corbett, and some sort of Northern Ireland connection between those two. We know that Hastings used to work in the Northern Ireland Constabulary thanks to a conversation he had in series 1.

The conversation revealed that Hastings had a male partner in the police who died in a pipe bomb attack, and whose death was later covered up (we don't know why).

Some fans are speculating that the dead police officer is Corbett’s dad. We know that Corbett was born in Northern Ireland, but he was later adopted into a Liverpool family (hence the scouse accent).

Could that be why Corbett was hell bent on getting Hastings pinned down as H as revenge for his (supposed) late father? And could that be why he attacked Hastings’ wife Roisin (Andrea Irvine) in the style of a Northern Irish paramilitary attack? Hopefully this will be revealed in the series 5 finale, or even in series 6.

Also, in slightly lighter news, there’s Arnott’s bad back. We see him in series 5 episode 4 struggle to perform when in bed with his ex-girlfriend, fellow copper Sam Railston (Aiysha Hart), who quips that it might be because of his injuries.

Loyal Line of Duty fans will remember that Arnott was briefly in a wheelchair in series 4 after being thrown down the stairs by a balaclava man, and he’s still suffering some back pain even after making a good recovery. Or has he?

The hidden romances

Line of Duty romances BBC One

In the opening episode of Line of Duty season 5, we see Arnott scrolling through a dating app on his phone before stopping at the profile of Tina Watts, whose photo on the app is a picture of her with a dog, above.

The woman in question is actually actor Compston’s real life wife, Tianna Chanel Flynn, and the dog is the couple's staffie, King.

Compston later shared the original image on his Instagram page:

Speaking of Arnott, aside from his bad back getting in the way of their sex life, his relationship history with Sam Railston goes back further than just series 5.

Sam is part of the murder squad and her work is intertwined with a number of AC-12's cases. She first appeared in Line of Duty back in series 3, when she was in a relationship with Arnott.

The pair split up after Arnott was framed and locked up as the Caddy by Cottan, and Sam believed that there was something going on between Arnott and Denton.

The former couple have been on speaking terms since, but ladies’ man Arnott hasn’t wanted to commit to her, relationship-wise, despite her advances.

Some fans are speculating that Sam’s supposed keenness is something more sinister, and she’s been using her relationship with Arnott to get closer to AC-12’s cases.

Remember the head of the murder squad was Detective Chief Superintendent Lester 'Les' Hargreaves (Tony Pitts), who was revealed to be bent in series 5 - although Sam insists she had no idea he was corrupt.

Moving onto Kate Fleming’s relationship with her husband, Mark. Fans who have just seen series 5 of Line of Duty might think that the couple enjoy a fairly harmonious family life (the couple share a son, Josh).

But viewers of the previous seasons will know that the couple have had nothing short of a rocky marriage, with Fleming having an affair with Richard Akers and Mark changing the locks on their family home at one point (Fleming had to live in her car).

In series 5, it looked like the couple were back on track as they watched their son sleeping soundly, but we’ve recently seen that all is not well between the pair, with Mark snapping at Kate for her late hours. Is there something else she could be hiding?

Things aren’t exactly on the up for Ted Hastings and his estranged wife, Roisin, either She’s featured quite heavily in series 5 after being attacked by Corbett, but the Hastings’ marriage problems go back much further than that.

Roisin and Ted had a long marriage and met in Northern Ireland, but financial problems and Ted's dedication to his work appears to have ruined their relationship.

She has filed for a divorce - another reason why unlucky Ted is living in a hotel, aside from his financial problems.

From Hargreaves to Hilton - All the H’s

Line of Duty who is H BBC One

If you’ve just started watching Line of Duty, you’d be forgiven for thinking that all the evidence points to the bent copper at the top of the ranks, known as H, being Hastings.

But fans of previous seasons will know there have been more characters under the H microscope than just gaffer Ted.

The copper that got killed at the Eastfield Depot in series 5 episode 3? That’s Les Hargreaves. The boss of the Murder Squad first appeared in series 2 when he brought Arnott in for questioning after the death of his AC-12 colleague Georgia Trott.

Hargreaves was a candidate to be H, but despite being proven to be dodgy and getting exposed in season 5, it was also confirmed that he was not the top dog in the criminal organisation.

How about Derek Hilton (Paul Higgins, above)? You’ve probably heard him referenced many times in season 5, and his mugshot is plastered on the wall in AC-12, along with the rest of the possible H’s.

ACC Derek Hilton was undoubtedly a bent copper, placing Dot in AC-12 and blackmailing Maneet Bhindra for information on the anti-corruption department.

But, with Hilton dead and H still at large, he's also been ruled out.

Then there’s Tommy Hunter. First appearing in series 1 as a criminal alongside little Ryan and the OCG gang, there was a theory that Hunter could be H with his position at the head of the balaclava gang.

But as the web of corruption was further exposed and Hunter was killed, it became clear he was a pretty minor player in a much more wide-ranging game.

And where did ‘H’ as the codename even come from? Going back to series 3, bent copper Cottan was shot and killed, and in his dying declaration he heavily hinted that the master of puppets had the initial ‘H’.

Although some people, including yours truly, have questioned the validity of the codename even being H at all…

Mother of God! The Hastingsisms

Line of Duty Hastings BBC One

Hooked on Hastings’ one-liners? The Northern Irish copper’s quotes have garnered an army of fans and earned him legendary status, although it’s not the first time he’s had such a distinct turn of phrase.

Since series 1, Hastings has tended to call the lads of the force "fella" and the ladies, "darlin’" (the latter getting him into trouble in the past - he’s not quite clued up on sexism in the workplace is our Ted).

He also generally calls any police man or woman a "copper", as opposed to, well, anything else:

He tends to use the phrases "wee girl" or "wee lad" as a term of endearment, usually when someone's naive or innocent.

In an interrogation setting, he generally repeats what the (probably bent) copper has said and replies, in a fairly aggressive manner, “I’ll give you [insert excuse here]”.

Hastings also has a penchant for a religious turn of phrase, and has long preferred exclamations including “Mother of God”, “Jesus, Mary and Joseph”  and “There but for the grace of God” when he’s surprised by something - usually some sort of evidence that his “best team” Arnott and Fleming have given him.

Series 5 viewers might look fondly upon the phrase “Now we’re sucking diesel”, but he’s said it long before this series. Translated to mean something along the lines of “Now we’re getting somewhere”, Hastings first said it to Fleming and Arnott in the last episode of series 4 after a breakthrough in the Roz Huntley case.

But the best one-liner to date has to be from series 5 episode 1, when he tells Detective Superintendent Alison Powell that he “didn’t float up the Lagan in a bubble”.

Meaning something along the lines of "I’m not stupid, darlin’’, Hastings is referring to the river Lagan, the river that runs through Belfast, and not a lagoon, and some fans speculated.

Now we're sucking diesel!

Catch up on Line of Duty now - seasons 1-5 are available to stream on BBC iPlayer.