Line of Duty’s sixth season didn’t break stride as it went into its second episode earlier this evening (28 March).

Last week, Jed Mercurio’s hit BBC thriller got viewers caught up with Steve Arnott (Martin Compston), Kate Fleming (Vicky McClure), Ted Hastings (Adrian Dunbar) et al, and threw a handful of new players into the mix.

Among these were Gail Vella (Andi Osho), the outspoken journalist whose murder sits at the centre of the new series, and DCI Joanne Davidson (Kelly Macdonald), the enigmatic murder investigator with something to hide.

Episode two also saw the return of a few familiar faces from previous seasons, including Steph Corbett (Amy De Bhrún) and Ryan Pilkington (Gregory Piper).

Fleming, meanwhile, has her loyalties tested after defecting to the Hillside Lane police station – working under Davidson in the very unit AC-12 is investigating.

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Here’s a breakdown of some of the biggest talking points and mysteries to emerge from Line of Duty series six, episode two.

Heavy spoilers follow for the latest episode of Line of duty, so be forewarned….

So much for Carl Banks

When Terry Boyle (Tommy Jessop) was arrested last week as a suspect in Vella’s murder, it seemed like an obvious stitch up – despite the clues pointing to Boyle’s unhealthy obsession with the reporter. Much more likely was Carl Banks, whose fingerprints were also found all over the apartment belonging to “Ross Turner”.

Banks proved quite the literal dead end, however – showing up with a slit throat near the episode’s beginning. The murder weapon was discovered near the scene of the crime, with the dead CHIS’s fingerprints on the handle. Could the CHIS have killed Banks before throwing himself off the car park roof? It all seems a little too neat – and the presence of the dodgy Ryan Pilkington, now a copper himself, at the crime scene seems to suggest there’s more at play.

What was Gail Vella working on?

While AC-12 doesn’t seem all that much closer to discerning the true killer of Gail Vella, they’ve dug up some interesting leads about her past. Speaking to a former co-worker, Compston and DC Chloe Bishop (Shalom Brune-Franklin) learn that Gail was working on an explosive podcast serial which would bring some scandalous information to light – seemingly concerning a link between high-powered police officials, politicians and crime rings.

What’s more, it seems that her house was burgled when she was killed – with a dummy laptop containing important files switched in to avoid suspicion. According to Vella’s former producer, police were told about the potential burglary during the initial investigation – but AC-12 found no mention of it in the original report.

Which side is Fleming on?

After appearing chummy enough with Arnott last week, Fleming steels up this time, tipping Davidson off about the impending AC-12 investigation. Davidson manages to undermine Arnott and Hastings when AC-12 comes knocking after Banks’ death, but only for a short while – she’s called in for an interrogation and the murder squad is brought under renewed scrutiny. Fleming meanwhile, earns her boss’s favour, and the scorn of Arnott and Hastings, both of whom immediately clocked onto her role in the tip-off.

When Fleming goes out for a drink with Davidson, it’s hard to miss the romantic undertones – could something be brewing between the pair? As last week’s episode showed, Davidson clearly doesn’t have a problem dating one of her subordinates.

Arnott moving on?

There’s no real development this week on Arnott’s request to move away from AC-12 – though perhaps Hasting’s decision to promote him to temporary DI could give him reason to to stay.

Arnott is also asked about the picture of the mysterious “fourth man”, the last missing piece of the “H” puzzle. This series has, so far, put the mystery of “H” on the back-burner and shifted its focus to Vella, but – despite Arnott’s insistence that he’s “moved on” – it seems like only a matter of time before the mystery fiend enters the picture once again.

In his private life, things are looking considerably bleaker for Arnott. His reliance on prescription painkillers seems to be getting the better of him, and this episode sees him dose up while on the job. A visit to Steph Corbett, the widow of last season’s renegade undercover officer John Corbett, sees Arnott break down in pain. As he bids goodbye to Corbett, the pair share a moment; you’d be surprised if their paths didn’t cross again.

Jatri stitched up

The episode’s final set-piece sees Davidson face interrogation and arrest, with her actions during last week’s armed robbery picked apart by Arnott and Hastings. Her communications records turn up nothing solid, but Hastings says they’re preparing to search her house for a burner phone. As she’s arrested, she reels off a list of other potentially crooked coppers who were positioned to sabotage the investigation: the CHIS handler, officers Farida Jatri (Anneika Rose) and Chris Lomax (Perry Fitzpatrick), and DS Ian Buckells (Nigel Boyle).

She has a point, and so AC-12 search all the potential suspects’ properties – finding a motherload in the residence of Jatri. A selection of highly suspicious burner phones seems like a strange piece of evidence to leave lying around – unless, of course, they were planted there by Pilkington, as the show strongly hints.

Released from AC-12’s custody at the episode’s end, Davidson gets into a car driven by none other than Pilkington himself, and coldly proclaims: “That’s what happens to a rat.” We are then treated to another mysterious sequence in which Davidson picks up a box containing a damaged mobile phone, before she breaks down sobbing in her car. It’s pretty clear by this point that Davidson’s not on the level, but there’s still a whole lot we don’t know about this back-stabbing DCI.