First-look Vigil pictures hint at shock ahead for DS Longacre

Presumably, that means viewers can expect another big revelation in the coming episode and, judging from Kirsten's facial expression, it looks like it could be promising news.

Meanwhile, we can also expect to see more of politician Patrick Cruden (Stephen McCole), with another first-look image showing him looking slightly concerned.

Cruden played a big role in episode three, during which it emerged Jade Antoniak (Lauren Lyle) was his secret daughter and that she had been feeding him information from Craig Burke pertaining to the submarine's failings.

As for what might happen on board Vigil itself, some further images give us a few hints about what to expect.

One of these sees DCI Amy Silva (Suranne Jones) being attended to by Tiffany Docherty (Anjli Mohindra) in the medical wing.

The final moments of episode four saw Amy discover the body of the boat's chef Jackie Hamilton before a masked Glover (Shaun Evans) appeared to rather forcefully remove her from the scene – could she be dealing with some medical issues after being exposed to something dangerous?

Apart from anything else, it promises another intriguing exchange between Amy and Tiffany, who haven't been on the best of terms since the revelations about Glover's secret relationship with the latter came to light.

Speaking of Glover, another image released by the BBC shows him having a conversation with Captain Newsome (Paterson Joseph), and it's safe to say that the coxswain looks a little troubled.

Newsome has had Glover's back up to this point – but could that be about to change?

Whatever happens, it looks set to be another dramatic and intriguing episode of the submarine drama as it nears the end of its brilliant six-episode run.

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.