Vigil episode 4 review: More revelations and another death

We begin this episode with a flashback (more on this later), before a terrified Amy Silva awakens to discover that the boat has received firing orders and is preparing to launch missiles. It soon becomes clear, of course, that this is just a drill – but not before Amy has become extremely distressed and sustained an injury while frantically trying to work out what is going on.

It does seem bizarre that the crew wouldn't have given Amy an advanced warning about the drill, but this isn't the first time she's been undermined during her time aboard Vigil, and it won't be the last either. Indeed, as this episode progresses it seems that Captain Newsome is especially keen to gaslight her – presumably in a bid to protect his crew, especially Glover who has been particularly under suspicion of late.

"DCI Silva I can't have you in questionable health, disrupting drills and making wild accusations," he says before relieving her of her duties and confining her to quarters. Now, I still think Newsome is a little too obvious a culprit to be the real big bad, but these are hardly the actions of a man with nothing to hide.

The biggest storyline on the boat this week involved the submarine's chef Jackie Hamilton – a character who we'd seen relatively little of up to this point, beyond her rather pronounced display of grief at Craig Burke's memorial last week. The episode ended with her becoming the third character in four weeks to bite the dust – and the second on Vigil itself – but the reasons for her death are not immediately clear. The stains Amy had earlier found in her bunk appeared to implicate her in Craig Burke's murder, but if that was the case then why has she now been killed herself?

Anyway, the most dramatic revelation of all this week didn't occur on the boat, but on land, as Kirsten – working together with both the Navy and MI5 – discovered that there is a Russian agent on board Vigil. After last week seemed to have ruled out Russian involvement, with the news that the boat that had been tracking Vigil had actually been American, this blows up the whole investigation once again: they're not just looking for a murderer, but a spy. It does all run the risk of perhaps becoming a little convoluted, but I think the writers have done a good job of keeping the tension up while juggling all the various plot points, and I still have faith that we'll get some satisfying answers in the final instalments.

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On the less positive side, this episode was also one that was extremely heavy on flashbacks – not about Amy's accident this time, but about her relationship with Kirsten. I've previously expressed some reservations about these flashbacks and this week underlined that. I felt these scenes did little other than disrupt the rhythm and tension of the episode, without divulging anything particularly interesting about either Amy or Kirsten. Clearly, the relationship is important to the series, if nothing else for their shared references that allow them to send and receive coded messages – but I'm not all that convinced that we gain a lot from seeing these fairly cliched scenes of them getting together in the first place.

On the whole, though this was another thoroughly engrossing episode of Vigil, and one that poses all sorts of very intriguing questions going into the final two instalments. Presumably next week we'll begin to tie up some of these many loose ends.

Vigil continues on Sunday nights at 9pm on BBC One and BBC iPlayer. For more to watch, check out more of our Drama coverage or visit our TV Guide to see what’s on tonight.