Your Line of Duty episode 4 questions – answered

But Line of Duty has never spoon fed its audience and as Roz wriggled out of his grasp, there were plenty of plot twists that we've had to rewatch to make sense of. Like...

How did Roz know that Kate was from AC-12?

Roz's suspicion of Kate was first raised last week when her loyal sidekick Jodie informed her boss that she'd seen their newest DS sniffing around the forensics office, making enquiries about a sample – KRG13. You'll probably recall that's the very same sample that Roz tampered with earlier in the series, switching it for a swab of blood from Timothy Ifield's bloody forensics suit after she realised the original sample might incriminate her.

Roz procured Jodie's mobile phone picture of Kate's enquiry and then got a further snap of her chatting with forensics investigator Kevin Gill in the car park, bringing both photos to her interview at AC-12 and accusing Ted Hastings and his team of tampering with evidence, to which he barked back: "my officers do not coach witnesses".

"So she is your officer," retorted Roz, smug in her confirmation that Kate was working for the other side.

Dare we say a quick Google of Kate and she'd have had her answer – Kate won an award at the end of the last series for her undercover work. It would have been a great deal simpler.

What was Roz trying to get Kevin Gill to do? And what did Kate say to him?

Before Kate had her chat with Kevin Gill, Roz was outside in the car park (above) bending his ear. She tried to get him to admit he'd overwritten the suspicious KRG-13 label or that someone else had tampered with it; he refused, telling her he wasn't going to be a whistleblower. But she must have got to him eventually as she came to her interview also armed with a statement from Kevin accepting responsibility for the rewritten reference – a version of events at odds with his response to AC-12 where he said he wasn't able to account for the crossed out label.

As for what Kate was saying to him, well, we never found out. Roz used it to unmask her DS as an AC-12 operative, suggesting she was trying to coerce Kevin, but that's unlikely. Perhaps Kate was trying to find out just what Roz was saying to him? Perhaps she was trying to find out whether he knew anything about the sample being tampered with?

Either way, Kate's attempts to trace Roz's footsteps proved the final undoing of her undercover stint.

How did Roz turn the tables on AC-12?

Just when Hastings thought he had her pinned to the mast, Roz wriggled free thanks to some inside information on AC-12 and a belligerent assassination of the Superintendent's character.

The early stages of the series saw a few clunky sexist lines from Ted who called Roz "darling" and chose to promote Steve over Kate. Episode four saw Roz capitalise as she accused Ted of sexism, citing his penchant for investigating senior female officers. She even knew Hastings was a mason and had the lowdown on the decision to release DI Lindsay Denton (Keeley Hawes) – a verdict overturned after questions were raised over the conduct of AC-12 officer Steve Arnott.

Hastings' assurances that his unit was not corrupt were quashed in one fell swoop. But how did she get her information?

The answer: Maneet! AC-12's best, most upstanding officer turned out to be, well, not so upstanding. The closing moments of episode four saw a sheepish PC Maneet Bhindra passing information to ACC Hilton. She was the leak!

What did Maneet give Hilton?

We can only assume that Maneet has been passing over details of her team – details that can be held over their heads in interviews like Roz Huntley's. We know Hilton is no fan of Hastings and his team, choosing to reinstate Roz despite their recommendations. But it's Roz who is AC-12's most sinister opponent; Hilton is mere putty in her hand, giving her written authority to prevent any further investigations into her conduct. And in her war against the anti-corruption team, it is thanks to Maneet's information that Roz now has the upper hand.

How did AC-12 know someone had tampered with the KRG-13 sample?

This is where things get complicated. Re-analysis of KRG-13 confirmed that there was more than a 99.9% chance that the blood in the swab was Tim Ifield's. So far, so good. But further tests also revealed there were a number of fibres from a forensics suit mixed up in the sample. Roz suggested they could have come from the forensics investigator who collected it – Kevin Gill – but Hastings dismissed her guess. An experienced FI would be trained to make sure that didn't happen and his suit had been specifically designed not to shed fibres that might corrupt evidence.

So, AC-12 concluded, the sample must have been taken from Tim's body suit – the body suit that the killer swiped from his flat on the night he died. Hastings' theory is that the killer – or someone close to them – was responsible for tampering with the sample. They are the only one who had access to the sort of swab that is now in the KRG-13 evidence bag.

Spot on, Ted. That's just what Roz did. But good luck proving it...

Why do AC-12 suspect Nick of killing Tim Ifield?

Two crucial pieces of evidence point to Nick (Lee Ingleby) as the Big Bad behind both Operation Trapdoor and Tim's killing. First, his car which was spotted heading towards and away from Tim's flat around the time of his murder. Of course, we viewers know the reason behind this – Nick was following his wife, Roz, whom he suspected of having an affair – but AC-12 don't.

Most of their suspicion hinges on the attack on Steve Arnott within Nick's building. If you recall, Steve was on his way to visit Nick to question his wife's version of her whereabouts the night of Tim's murder. He got into the lift of his building and headed up to see him on floor five but the lift stopped at the third floor and Steve was brutally beaten by a man in a balaclava.

Balaclava man (for now let's assume there's just one) was spotted on CCTV moments before, heading into Nick's building. Did he tip him off? You can certainly see why AC-12 think Roz is trying to cover for her husband.

Did Roz have an affair?

No. It's a story she concocted to explain to her husband why she was in the vicinity of Tim's flat during his murder. Why he believed that his wife – who's been acting strangely and keeps rubbing her wrist – was in the same building as Tim at the time he was killed but didn't have anything to do with it is beyond us. Their dynamic is undoubtedly weird and Nick isn't exactly lapping up Roz's explanation without expressing his reservations – but he hasn't forced a confession from his wife.

Is Nick actually bad?

We're about as confused as you are. It would be a strange move if the series suddenly revealed that husband and wife were in league with one another after everything we've seen Roz hold back from Nick during their conversations at home. But Mr Huntley still seems rather suspicious. After all, Steve was attacked in his office building and Nick was the only one who knew he was there.

And then there's that final scene between Nick and Roz – the one with sinister music playing as he lurks behind her in their darkened hallway. For a split second we thought he was going to kill her... but then she wandered off towards bed as he hovered in the shadows. It's like the Line of Duty editors are trying to tell us he's a wrong 'un. Whether he turns out to be or not, we'll have to wait and see.

This article was originally published in April 2017