Line of Duty: 5 unanswered questions we're obsessing over after episode 1

And another big revelation: Stephen Graham's "balaclava man" John Corbett is an undercover police officer! But is he still loyal to the police, and what is he playing at? The plot thickens.

• Meet the cast of Line of Duty series five
• Line of Duty spoiler-free review: a triumphant return – with a menacing new villain and plenty of twists and turns
• Listen to the RadioTimes.com Podcast now: subscribe on iTunes / subscribe on Google Podcasts

Here are the five big questions we're puzzling over after episode one (WARNING: SPOILERS)...

1. Is Hastings "H"? Was Hilton really guilty?

Superintendent Ted Hastings (Adrian Dunbar) remains utterly convinced that ACC Derek Hilton (Paul Higgins) was "H" – but the more he insists that Hilton is the ultimate top dog, the less inclined we are to believe him. The same goes for DI Kate Fleming (Vicky McClure), who is clearly having grave doubts over the gaffer's judgement.

"It's been one hell of an journey to this point, hasn't it?" Hastings asks AC-12 as he presents the pin board with a printout of Hilton's face on it under the letter "H". "Small mercies, Maneet's confession has put the final nail in Hilton's coffin. Hilton knew about Malhotra, oh yes – he was the one. He was the senior police officer who was running the show for organised crime. Killing himself was the only decent thing that man ever did. H, finally. Well done everybody. Well done."

But when DS Steve Arnott (Martin Compston) suggests that only the undercover officer in the balaclava gang would be able to prove the gaffer 100% right, Kate adds: "Or prove him wrong."

While plenty of evidence now points to Hilton's collusion and corruption, it's by no means clear that he was the police insider who ultimately controlled this organised crime gang. It's also weird that Hastings has accepted Hilton's death as "suicide", even though he died on the very same spot where series three's social worker and whistleblower Oliver Stephens-Lloyd was murdered in another staged "suicide." (And now Maneet's also turned up at that spot, it's even clearer than ever that Hilton was killed by the same organisation.)

So... is Hastings actually the real "H"? And... is that why he's pinning everything on Hilton?

Writer Jed Mercurio has been teasing us with this ultimate Line of Duty conspiracy theory since the end of series four, and now he's ramping up the suspicion. In episode one we see Lisa McQueen's phone ring on her passenger seat as an "unknown number" flashes up; she ignores it. Meanwhile, Hastings is shown calling someone who fails to pick up. Was Hastings ringing Lisa?

2. Maneet is left for dead – but how did that bug end up in her burner phone?

Line of Duty has a reputation for dramatically killing off key characters, and now it's time to say goodbye to Police Constable Maneet Bindra (Maya Sondhi). The cutthroat balaclava men dramatically cut her throat in the final seconds of episode one, leaving her to die on the jetty as her blood poured down between the wooden slats.

To be fair, we probably should have seen the writing on the wall when she showed Kate a video of her two adorable kids. She might as well have said she was a cop on her last day before retirement.

In the AC-12 interrogation room, Hastings and co finally find out that Maneet was the one leaking info to ACC Hilton (and had pinned the blame on DC Jamie Desford by using his computer). Hilton's leverage was Maneet's cousin Vihan Malhotra (Maanuv Thiara), who was in danger of dismissal for a series of mistakes.

"It's true ACC Hilton used Vihan to blackmail me... Hilton said he'd pull strings if I kept him in the loop," a tearful Maneet finally admitted. "Then he put more pressure on me to reveal more and more sensitive material."

Either independently of this, or deeply connected, was Vihan's position as a police insider for the balaclava gang. He worked as a civilian administrator within the police and was involved in organising the processing and disposal of controlled substances, which is why they targeted him for this hijack of the ED905 drugs convoy; by pretending to help him consolidate and pay off his gambling debts, Lisa McQueen (Rochenda Sandall) gained enough information to blackmail him into passing on the relevant information. The £25,000 in cash also helped.

But despite Maneet's best efforts to keep her beloved cousin off the radar, Vihan's actions were discovered and he ended up behind bars pending trial. At this point, Maneet seems to have decided to go full-on mole, approaching the gang and offering her services as a police insider in return for Vihan's protection inside prison.

"I passed information to ACC Hilton, I could do the same for you," she tells Lisa and gang member Miroslav (Tomi May). Is that a flicker of a reaction to Hilton's name? Or complete indifference?

But it all goes wrong for Maneet. Although Lisa is keen to collaborate and has her call in the tip-off about the Slater gang's drugs haul, John Corbett (Stephen Graham) does not trust her. He has her kidnapped – and when he opens up the back of her burner phone, he finds some kind of listening or tracking device on her microchip. Game over for Maneet, and bad news for Lisa who is in Big Trouble with her boss.

So who put the chip there? Was it John, keen to dispatch a cop who might find out his true identity? Did he plant the chip in the phone to discredit Lisa's judgement? It seemed like he knew exactly what he was looking for, and where. On the other hand (and this seems less likely), could Maneet have been trying to get inside the gang on behalf of AC-12 as a kind of double-bluff, and we just don't know about it yet?

• For the latest news and expert tips on getting the best deals this year, take a look at our Black Friday 2021 and Cyber Monday 2021 guides.

3. Is Stephen Graham's undercover cop John Corbett corrupt or not?

Steve and Kate spent the whole episode assuming that Lisa McQueen was the UCO (undercover officer) operating inside this OCG (organised crime group), but then – surprise, surprise – it turned out she was just a bona fide criminal. Her wobbles over shooting a female police officer in cold blood had nothing whatsoever to do with being an undercover cop. Perhaps she's just got a little too much of a conscience.

Instead, the undercover officer is Stephen Graham's character, the balaclava gang's menacing leader John Corbett.

So what are the implications of this big twist?

Despite her extreme reluctance, Hastings finally drags some details out of Detective Superintendent Alison Powell (Susan Vidler). Corbett was embedded as part of Operation Peartree, but it's been months and months since either the comms or the cover officers lost contact with him. "We can't be certain what happened, it's a deep cover operation," she says.

It's so secretive its files are protected on the police database by a Management of Police Information (MOPI) notice. "I've been brought in from an outside force to lead on a specific operation authorised at the highest level, and we've done a good job of keeping it hidden," she says. And "our operation is at a critical stage, best served by leaving our asset in place."

At this point, has Corbett gone rogue? What game is he playing? And who is he really working for?

4. Why did Kate get promoted – not Steve?

"Steve, are you okay to open a case number?"

"Ma'am."

Oof. That's how we find out that Kate Fleming is now a Detective Inspector, while Steve Arnott remains a Detective Sergeant. Kate used to be his junior – and now she's "one rank superior."

Back in series four, it looked like Steve was being lined up for DI. So what changed in the two years since we last visited AC-12? Hopefully we'll find out...

5. What's going on within the balaclava gang?

Corbett seems to be calling the shots and has his own office in the back room of Kingsgate Printing Services, the gang's HQ. But Lisa clearly holds an important role, perhaps as his second in command.

If this is the same organised crime group which has been running for at least eight years, what is the history here? How did each of them rise to power, and why does Corbett say he's fed up of giving Lisa second chances?

Line of Duty continues on Sundays at 9pm on BBC1