Incredibly we’re already at the halfway stage of Vigil – and the submarine-set series continues to stake its claim as one of BBC One’s best new thrillers in years. Tonight we were treated to another riveting, perfectly-paced episode that saw several new secrets come to light, continuing to ramp up the tension as Amy and Kirsten attempt to solve just what’s been going on in the titular submarine – and in the navy at large.

There was also a lot of information to digest during the episode – relating to events and characters both on land and at sea – as the significance of a past stop in Florida became increasingly clear and some personal revelations about a number of characters thew the investigation wide open.

Read more: Vigil: 7 big questions we have after episode 3’s big reveals

The episode began on land, as we dealt with the fallout of Jade’s tragic death at the end of the second instalment. Through this, we were introduced to a couple of new characters – anti-Trident politician Patrick Cruden (Stephen McCole) and his assistant Mark Hill (Oliver Lansley), the former of whom played a significant part in the events of the episode. Initially, Kirsten was stumped as to the nature of the relationship between Cruden and Jade, but towards the end of the episode, he revealed that she was his secret daughter, and he had set up a company to send her an allowance. He also spilled some key information relating to Jade and Burke’s whistleblowing activities: Burke had been on to something relating to a key event that had resulted in two deaths and an officer being sent away to the Middle East as part of a cover-up.

That incident is clearly going to have a huge impact on the investigation, and Amy finds out plenty about it herself during an encounter with Gary Walsh (Daniel Portman) towards the end of the episode. We’d seen a death per episode in each of the first two instalments and for a brief moment, it looked like Walsh was going to be the next poor soul to bite the dust, as he stood with a gun to his head in a terrifically tense scene before he was talked out of pulling the trigger by Amy. Suranne Jones has been tremendous in the series up to this point and she was at her very best during this scene as she detailed her tragic past, which had so far only been glimpsed in flashback. If I’d had one reservation about the series in these early episodes it’s that perhaps Amy’s traumatic backstory had been over-egged a little, but the way she was able to draw on her own experiences to address Gary in this scene was brilliantly done.

Shortly afterwards, Gary revealed some more key information – explaining that two local contractors had been “cooked alive” when Vigil had been in Port Havers for missile servicing, after a junior mechanic had made a major error with the reactor upon returning from a night out. According to Walsh, had the error not been caught when it did, it could have resulted in a nuclear disaster on the scale of Fukushima, while he also revealed that all those who had known about it had been told to keep schtum. The evidence of a major cover-up is only continuing to mount, and it seems clear that whoever dispatched Burke was someone at the very top of the chain – or at the very least someone following orders from the very top of the chain.

That wasn’t the only new evidence revealed during the episode, either: despite having seemed two of the more friendly faces on board Vigil in the opening episodes, Tiffany and Glover are both now firmly under suspicion after their secret relationship was discovered, while we were also given the bombshell news that the boat which had been covertly following Vigil had not been Russian, as they’d suspected, but American. And then there was the closing scene, in which Kirsten was followed to a multistorey car park only for it to be revealed that her trackers were none other than MI5 – seeming to confirm Jade’s theories that she had been being watched. The way in which writer Tom Edge has been able to weave together the two strands – the submarine scenes and Kirsten’s investigation on land – has been nothing short of superb so far, and it will be extremely interesting to see just how all these events coalesce in the remaining three chapters.

Since the very first episode, the series has also made excellent use of its submarine setting, and this week we got the rather bizarre scene of Burke’s memorial taking place on the boat. There was something strangely haunting about the experience of hymns being sung by the boat’s crewmen, and it made for another wonderfully unique scene in a drama that has already marked itself out as different from just about any other crime show on TV.  Before the series started, Edge spoke about how the “mythical” nature of submarines brought to mind ancient stories and epic seafaring sagas, and there was certainly something of this in that memorial scene. Of course, it also offered us – and Amy – the rare chance to see most of the key personnel in one place, and it was very interesting to study the facial expressions of the various crew members as Captain Newsome (Paterson Joseph) delivered an address and reminded the crew of their task at hand.

With just three episodes to go until we (presumably) get to the bottom of who killed Craig Burke (Martin Compston) there are now more questions than ever before – but given the quality of the series so far, we have full faith in the remaining chapters to give us satisfying answers.