Line of Duty review: Who killed Gail Vella and the CHIS? Where have we seen Buckells, Terry Boyle and and Farida before? And what is Kelly Macdonald’s Jo Davidson up to?

The idea of watching TV shows week by week may be deemed pretty old fashioned these days, but if anyone tried to stick all of Line of Duty on iPlayer at once we’d riot.

Yes, we may be clamouring for answers after an opening episode that teased multiple new storylines and an intriguing central mystery. But the joy of having a whole week to ruminate various theories about who’s a wrong 'un, who’s bent and what Kelly Macdonald’s DCI Jo Davidson is up to far outweighs the frustration of waiting seven days for the next instalment.

Plenty has changed since the end of season 5 – Kate Fleming (Vicky McClure) is no longer in AC-12, apparently departing with a slight bit of acrimony due to Ted Hasting's (Adrian Dunbar) laptop antics and shiftiness with John Corbett (RIP).

She’s now working at Hillside Lane Station alongside DCI Jo Davidson and under the not-so-watchful eye of DCI Ian Buckells (Nigel Boyle) – a familiar face for long-term fans of the series.

8 things you probably didn't know about Line of Duty

Like Line of Duty? You'll love these shows

Steve Arnott (Martin Compston) is growing increasingly frustrated that his dazzling waistcoats haven’t earned him a promotion yet and appears to be over-doing some serious looking painkillers. He's also angling for a job with his ex Nicola Rogerson (Christina Chong), which looks like it will end badly.

And poor old Ted is still getting the cold shoulder from all sides because of the John Corbett affair. DCC Andrea Wise and the higher powers are blocking him from meetings and it takes some serious digging from Steve to get the gaffer’s ‘bent copper’ radar back up and running.

The crime at the heart of season 6 is the murder of journalist Gail Vella, shot by a professional at close quarters in a car park. It looks like Gail’s investigations were getting a little too close to the OCG's antics and corruption in the police force.

Confused? Here is everything you need to know about episode 1 and all the burning questions…

Line of Duty season 6 - Secrets from the Set

Who is Jo Davidson and what is she up to?

The pre-show hype from Jed Mercurio teased that Kelly Macdonald’s character would be AC-12’s “most enigmatic adversary yet”. And he wasn’t overselling it.

The Scottish actress follows in the footsteps of guest villains Thandie Newton, Lennie James, Stephen Graham, Keeley Hawes and Daniel Mays, but despite the levels of expectations, DCI Jo Davidson didn’t disappoint.

Friendly boss and thorough investigator one moment, suspiciously closing meeting room doors the next, Davidson is definitely playing games, but her intentions are still not clear.

Davidson is clearly running the Hillside Lane team – Buckells may outrank her, but she’s definitely got control – and she was involved in delaying the arrest of the prime suspect in the Vella case by diverting the team towards a conveniently timed armed robbery.

At home Jo isn’t the calm and poised figure she cuts at work. She has approximately 27 gigantic locks on her frontdoor and is bubbling with fury at a picture of what we presume is her as a young girl with her mother. When she smashes a drink and screams, we’re left to wonder whether it’s a howl of anguish or anger.

Is Davidson working for the OCG? Could she be the fourth ‘H’? It’s still too early to call, but judging from her suspicious hand-rubbing and flirtation with Kate at the end, it looks like she’s definitely a game-player and plotter.

Who killed Gail Vella?

At the moment, it looks like OCG member Carl Banks is the prime candidate. Thanks to the hold-up caused by Jo Davidson’s antics and Buckles ‘bungling’ of the surveillance team, Banks had plenty of time to flee the building that the CHIS had given as a tip-off.

He left behind Terry Boyle to take the flak and waste some police time, but not even Davidson could pin this murder on poor old Terry.

Carl may be the one who pulled the trigger, but his reasons and the people making the order are who we’re really intrigued to discover.

Why was the CHIS killed?

The CHIS (that's a Covert Human Intelligence Source - we had to Google it) gave the Hillside Lane team their first serious tip-off in the Gail Vella case. The sex worker handed over an address of a person who was behind the shooting. However, when the raid on the address went wrong and the suspect escaped, the CHIS suddenly became the only person with knowledge of the shooter. A vitally important figure for the police, but also sadly for the OCG.

Before they could be persuaded to reveal any more information, their body was found, pushed off a building, conveniently away from any CCTV cameras.

Did Buckells and/or Davidson manage to get the CHIS’s identity and then have them killed? Or even do the deed themselves?  

What happened between Ted and Kate?

Kate is no longer at AC-12 and judging by the gaffer’s forlorn looks and sighs, things didn’t end happily between the pair.

Arnott is clearly close to his old buddy and wants to use Kate undercover to investigate Davidson, but Ted is more uneasy about working with her.

Splitting up the team was a smart move from Mercurio, throwing doubts into our minds about Kate and Ted that superfans will enjoy piecing together into elaborate H theories.

When it comes to the crunch will Kate side with AC-12 or Davidson? That moment is surely on the horizon.

Who is DCI Buckells?

Nigel Boyle returns for his third series in Line of Duty as the unpleasant and fairly useless DCI Buckells.

He originally appeared in season 1 as the senior officer who replaced Tony Gates and then again in season 4, taking over from Roz Huntley in the Leonie Collersdale and Baswinder Kaur murders.

A constant thorn in the side of AC-12, it’s interesting to see him working alongside Kate after yet another promotion to the role of DCI.

How does Buckells keep landing on his feet? Is he cunning mastermind or is he just someone willing to keep his mouth shut and look the other way to get promoted?

Who is Terry Boyle?

Terry (Tommy Jessop) is another character who has appeared twice before in the series and is back for a third run. Poor Terry first appeared back in season one, when his fridge was used by the OCG to hide the body of Jackie Laverty.

He returned in season 5 when it was revealed that Ryan Pilkington was still using and abusing Terry’s goodwill and residence to keep an eye on the OCG’s print shop.

Brought in for questioning about the Vella murder, someone attempted to set Terry up as the murderer. Luckily for him, no amount of bent coppers could make this case stick.

One thing we know about Terry is that whenever he appears in the show, Ryan Pilkington won’t be far behind. Last seen passing his police exams, we wonder how long it will be before we see his face again…

Who is Farida Jatri?

The final returning character in the episode was Sergeant Farida Jatri (Anneika Rose), last seen in season 4 working for Roz Huntley at Polk Avenue.

Now working at Hillside Lane, she contacts Steve to flag up Davidson’s suspicious behaviour in the Vella case.

However, there is more to Farida and Jo’s relationship than first meets the eye. Initially we’re led to believe that she fears Davidson and she calls Steve because she genuinely believes there something is amiss.

But it is then revealed that Farida has been in a long-term relationship with Jo. She believes that Jo has been cheating on her – even suspecting a relationship with Fleming – and is so paranoid she’s been cutting up Jo’s clothes to find evidence.

Will Farida’s personal conflict of interest jeopardise any evidence she gives against Jo? And how worried should we be that Farida won’t make it out of this series alive?

Who is H?

The new series posed so many new questions that the long-running mystery about the fourth bent copper didn’t get much of a look in.

Fans who believe the Kate and Ted theories were given fresh impetus, but it would be unfathomable and nigh on impossible for any of the lead trio to be involved as the fourth H this deep into the series.

We did get a glimpse of DCC Andrea Wise (Elizabeth Rider) snubbing Ted from the latest bigwigs meeting. Alongside PCC Rohan Sindwhani (Ace Bhatti), she must be somewhere near the top of the H suspect list.

But if we were going to place a sneaky bet on the fourth H, we reckon Nicola 'Jolly' Rogerson (Christina Chong), Steve’s ex-girlfriend, is a decent stab in the dark. Another recurring character, who always seems to be somewhere near the trouble, her flirtations with Steve at the coffee shop might be part of her plans to keep a close eye on AC-12’s investigations.

Watch Line of Duty season 6 on BBC iPlayer and new episodes every week on Sundays at 9pm.

Catch up on season 1-5 on BBC iPlayer