Vigil: 7 big questions we have after episode 3's big reveals

Meanwhile, the episode also included the uncovering of important secrets relating to crew members, a major bombshell about the submarine that had been tracking Vigil and some intriguing involvement from MI5.

We've posed all the big questions we have after the episode below – read on for a full recap.

What is the significance of Glover and Tiffany's relationship?

Up to this point, Coxswain Elliot Glover and Surgeon Lieutenant Tiffany Docherty had appeared to be two of Vigil's more welcoming faces, but they've now been placed firmly under suspicion after Amy – thanks to a little help from Kirsten and the convenient placing of a topless photo of Glover – discovered their secret relationship. But what does this mean for the show – and why are they hiding their relationship?

The relationship comes to light when one of the files from Craig's USB stick shows photos of Tiffany with a man, although it's initially unclear who this man is, with the only identifiable feature being a dragon tattoo on his upper arm. For most of the episode, Amy is unable to decipher Kirsten's coded message – which says that while she has a weak visual on the man, he has "her favourite book." It's only at the end of the episode that Amy spots a photograph of Glover in which the tattoo is visible and is able to put two and two together, which then leads to a rather frosty exchange at the end of the episode.

Suspicion had fallen on Tiffany at various other points throughout the episode; towards the beginning, she is tasked with narrowing down the substance used to poison Craig – and she appears a little reluctant to look into it. Meanwhile, when Amy questions Prentice later in the episode, he reveals that the fudged drugs tests were likely falsified at the testing level, clearly implicating Tiffany, who had been responsible for carrying out the tests.

Meanwhile, it also emerges that Glover had known about Craig's claims he had been spiked and had withheld that information from Amy – claiming that Craig had "a whole list of petty grievances" and he doubts most of them were true. During this exchange, Amy says she reckons Tiffany has the means, motive and opportunity to have poisoned Craig, and she is now clearly a suspect.

When questioned, Tiffany reveals she did indeed swap some of the samples, but she claims she did so purely to look out for her friends and not because she was ordered to do so by higher-ups. Then, when Amy suggests that Tiffany could have poisoned Craig because he was blackmailing her about the photo, Tiffany seems genuinely unaware of the existence of the image – and so it admittedly seems unlikely that she was responsible for Craig's death, at least on those grounds.

However, Glover later reveals that he did know about the photo, which he claims that he only didn't tell Tiffany about because there was "so much going on." There is a clear implication that he's more involved than he's been letting on – especially when Tiffany promises him that she "won't say anything." So could it have been Glover that poisoned Craig? And what exactly will Tiffany not say anything about?

What exactly happened at Port Havers?

A good chunk of the episode was spent trying to work out who or what 'Davies' was – after a former Vigil crewmate instructed Kirsten to look into that name, without giving any further information. The police were unable to find any crewmate with that name, and it eventually turns out that it wasn't a person at all but rather the name of a naval services company based out of Port Havers in Florida. What's more, two of the company's contractors had died at exactly the same time that Vigil had been stationed there.

It turns out the deaths were the fault of a former Vigil mechanic named Ross Harmison, who upon returning drunk to the submarine had made a critical error with the reactor that almost led to a nuclear disaster. Although the worst possible scenario was avoided, the incident did lead to the aforementioned contractors being "cooked alive" – with the official reasons for their deaths given simply as drowning and Harmison sent to a different station in the Middle East. It's clear, then, that the Navy is covering up this incident – and the suggestion appears to be that Burke knew about this and was about to expose it, hence his murder.

Who spiked Craig Burke?

One of the key findings from the files on Craig's USB stick – which the police have now been able to access after Jade passed on the password just before her death – was a record of a drugs test that was carried out shortly after the submarine had docked at Port Havers in Florida, where 15 Vigil crew members were later arrested for being drunk and disorderly and the apparent use of drugs.

It turns out the hair sample Craig turned in had returned a positive test for LSD, which is directly contradictory to the official Navy tests, all of which had been negative. Initially, Kirsten and her team reckon that Craig was trying to prove someone else had been using, but it soon emerges that he was in fact aiming to show that he had been spiked by someone else.

But why was he spiked? If we are to assume that most of the key information that he was set to leak emerged directly from the events of Port Havers, is it possible that someone knew he was a security risk and spiked him to get him thrown off the boat and disgraced before he could reveal anything?

Why are the Americans tracking Vigil?

One of the biggest issues for the Vigil crew up to this point has been the very real possibility that they were being tracked by an enemy submarine – but after a tile from the submarine in question was used to identify the class of boat, a major bombshell was dropped: it's not the Russians who are tracking Vigil, but the Americans.

This leads to a very angry exchange between Rear Admiral Shaw and two MI5 agents – with the latter claiming they will ask for more information from the Pentagon immediately. But MI5 also claim they reckon the covert tracking is linked to the continued tension from the event at Port Havers. It seems likely the Navy had kept the real reason for the deaths of the Davies contractors from their American allies as well, so perhaps they are being investigated by the Americans on those grounds?

Or is there another reason the Americans are tracking them? We don't have an awful lot to go on just now, but the Navy clearly seems more concerned about it than MI5.

Who is leaking information to the press?

Relations between the police and the Navy have been poor throughout the drama, and another row emerges during this episode when Lieutenant Commander Erin Branning calls Kirsten and furiously demands her to turn on News 24. Upon doing so, Kirsten discovers that the media is reporting Burke's death – but she is unclear as to how they have got hold of the relevant information. Kirsten insists it didn't come from her end – suggesting that Craig's family could have been responsible for the leak – but Erin seems rather unconvinced.

So who is behind the leak? There are two likely motives for leaking: it could be someone who wants the Navy's failings out in the open, or it could be someone attempting to undermine the investigation. Either way, it adds yet another source of tension to the ongoing drama.

What do MI5 want with Kirsten?

The very end of the episode sees Kirsten attempting to escape a car that's on her trail, eventually leading to a reveal on the top floor of a multi-story car park. It turns out that it's none other than MI5 who are tracking her – the very same agents we have previously seen meet with both Shaw and Robertson.

So what do they want with her? Well between this and the earlier reports they had been spying on Jade at the peace camp, it seems clear that there's something related to Vigil that they don't want to come out. Could the intelligence services themselves even be responsible for the deaths of Craig and Jade? At this stage, it's certainly a possibility.

Who killed Craig Burke and Jade Antoniak?

Of course, the overarching question remains who killed Craig Burke and Jade Antoniak, and at the start of the episode, Kirsten and Detective Superintendent Colin Robertson agree that the two deaths should now be treated as one investigation.

We now know for sure that the pair had been collaborating to expose Vigil, and both were presumably killed to stop them from taking any of their findings public, so the assumption is that whoever is responsible holds a high position in the Navy. Of course, given that one death occurred on land and the other on the submarine, we know that they weren't killed by the same person, so it seems that there is rather a complex conspiracy going on.

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One theory that played a large part during the episode but which can presumably now be put to bed concerns the role played by politician Patrick Cruden and his staffer Mike Hill, who Kirsten learned had had some suspicious dealings with Jade. It turns out that Jade had been Cruden's secret daughter and that she had been feeding him information from Craig – but given the nature of their relationship, and given it is clear he also wants to expose the deterrents programme, it seems certain that he had no involvement in her death.

And then, of course, we have Glover and Tiffany. There are certainly now suggestions that one or both of them could have been involved in Craig's poisoning – but then how could they possibly have caused Jade's drowning? The most likely scenario remains that the conspiracy goes to the very top.

Vigil continues on BBC One on Sunday 12th September at 9pm. If you’re looking for more to watch, check out our TV Guide or visit our Drama hub for the latest news.