With the sixth series of Line of Duty approaching its midway point, it seems like things are starting to heat up for Steve Arnott and co.

Episode three features the most memorable set piece of the season to date, as nefarious officer Ryan Pilkington (Gregory Piper) attempted to drown a key witness and fellow police officer after a lakeside car accident. The scene embodies much of what makes Line of Duty great: it’s unexpected, edge-of-your-seat stuff that manages to sell a ridiculous twist as completely plausible. 

Elsewhere, the episode is more subdued, though no less tense. DCI Joanne Davidson (Kelly Macdonald) is proving a wonderfully slippery foil for Arnott (Martin Compston), and the pair’s investigational tête-à-tête is carried over from last week without missing a beat.

While various fan theories have already sprung up after the season’s first few episodes – such as the suggestion that Davidson may have ties to last season’s fallen officer John Corbett – it seems the series still has plenty of mysteries left in the tank.

It’s been theorised by some, including former Line of Duty star Craig Parkinson, that the show could kill off a major character in the next few episodes. For now, however, all the major players are still alive – just about.

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See below for our rundown of the biggest talking points and theories from Line of Duty series six, episode three.

It goes without saying, but heavy spoilers follow…

Boyle back in the frame

Terry Boyle (Tommy Jessop), the suspect with Down’s syndrome who was possibly identified as “Ross Turner” back in the season premiere, comes into Operation Lighthouse’s crosshairs again when witness Deborah Devereux (Kerri McLean) comes forward to corroborate Boyle’s alleged interaction with the deceased informant Alastair Oldroyd. 

Boyle is hauled in again and faces questioning by Fleming and Davidson. He gets increasingly distressed by the interrogation, and Fleming clearly thinks he is about to divulge some key information, but Davidson shuts down the interrogation, ostensibly for Boyle’s welfare and to safeguard the legitimacy of their case. It’s another instance of Davidson stalling the investigation when a breakthrough is imminent – and Fleming smells a rat.

Things get worse, before they get wetter

After Boyle is released, he is escorted home by Pilkington, and another uniformed officer, Lisa Patel (Tara Divina). Fleming, on a hunch, follows them. In a sensational turn, Pilkington attacks the other officer while she’s driving, sending the car speeding into a lake. Having escaped the car, he drowns Patel in a struggle, and starts to do the same to Boyle – before Fleming shows up and summons authorities to the scene.

Pilkington emerges from the incident a hero, even being put forward for a commendation later byDS Ian Buckells (Nigel Boyle). Fleming is onto him, however, and starts co-operating with AC-12, tipping them off to her doubts about Pilkington. Arnott and Fleming soon identify him as the malevolent juvenile offender from series one, when he once tried to cut off Arnott’s fingers with a bolt cutter. They suspect his record must’ve been hidden when he was vetted for the police force – but by whom?

While his status as a recent appointee to Davidson’s squad puts the DCI firmly in the spotlight, she later tells Fleming that Pilkington was in fact pushed through by Buckells, who ends the episode in handcuffs after the evidence starts mounting against him.

Along with the hiring of Pilkington and thedubious paperwork snafu that seemingly allowed the OCG to frame Boyle for murder, it turns out Buckells also has history with Devereux, the witness. It’s hard to see a way he can talk himself out of this, but it doesn’t seem likely he’s the season’s true villain – it all seems too neat a solution, especially with four episodes to go.

Old battles

Episode three did little to clear up the series’ bigger mysteries. AC-12 are given access to some of the material Gail Vella (Andi Osho) was working on before her death, and none of it was enough to get her killed. Perhaps the stolen files discovered in Buckells’ car at the end of the episode will prove more fruitful.

Simmering quietly on the backburner, too, is the search for “H”, now referred to, as Ted Hastings (Adrian Dunbar) points out, as the “fourth man”. He is told by the DCC to let the trail run cold, that he needs to stop fighting old battles. “The name’s Hastings ma’am,” he responds. “I’m the epitome of an old battle.” It’s looking like Hastings is on thin ice, as is – will the hunt for the “fourth man” end up being his undoing?

Arnott caught out

After Hastings is alerted to Arnott’s pill problem via a concerned phone call from Steph Corbett (Amy De Bhrún). Shortly after, a round of drugs tests are conducted in AC-12, which sends Arnott into a flash of panic – before Hastings said he had already got Arnott out of taking one. Arnott rightly interprets this as a “nudge” to sort himself out, and goes storming round to Steph’s with his grievance.

Arnott does manage to give up the pills, for a night at least, and returns to Steph’s for a night of emotional (but not sexual) intimacy. Lying next to her in bed, he tells her that his long-term injuries have left him with sexual problems. The next morning, Arnott waits til she leaves and conducts a search of Steph’s house, eventually uncovering the £50,000 of OCG money given to Steph by a guilt-stricken Hastings last season.

If that weren’t bad enough, Arnott finds himself turning back to the painkillers when booze doesn’t do the trick– and is blindsided by another drugs test, which he seems unable to get out of. What this means for Arnott’s job, and his relationship with Hastings, is still very much up in the air.

Chekhov’s freezer

Finally! The freezer in which Jackie Laverty’s corpse was stashed back in series one has finally been discovered (minus the body, of course). There’s enough DNA in the fridge, which was kept until recently in Terry Boyle’s residence, for the forensic team to identify Laverty, however, and Hastings and Arnott quickly begin piecing together some of what happened. “This is the whole damn thing,” remarks Hastings epiphanously, as he realises that Boyle must have been manipulated to help conceal the OCG’s criminal activity in a practise known as “cuckooing”.

Elsewhere, AC-12 have another strong lead in Farida Jatri (Anneika Rose), though a violent intervention from a prison warden saw Jatri sent to the prison hospital, refusing to answer questions. Despite the OCG’s best efforts, however, it seems like only a matter of time before the truth comes tumbling out.

Line of Duty continues next Sunday at 9pm on BBC One.