One story line was about how close to the end of life the nuclear subs are (Vigil being the name of the boat in the drama, but clearly meant to represent our current Trident subs). It also dramatised tensions between the US and the UK. In particular, a few US contractors died when Vigil was in a US port for maintenance work and the incompetence of a UK sailor led to their demise. Subsequently the US sent a sub to shadow Vigil on it's current deterrence-at-sea mission, which initially led the Captain to go all paranoid and believe the Russians were shadowing him.
A separate story line was the sinking of a Scottish trawler - Vigil heard this on sonar and worked out that it wasn't them so must have been another sub close by - hence the paranoia. The Admiralty eventually worked out it was a US sub, and discovered the lack of trust from our friends across the water.
The main story line was a british sailor seduced by a Russian agent into being a traitor, and to actively sabotage Vigil to force it to surface near Russian ships as a propaganda coup. The background context being the upcoming decision in Parliament about renewing the at-sea nuclear deterrent.
The final chapter of the drama had the whole traitor/sabotage story covered up, with a Scottish politician chatting with the Admiral and agreeing to keep the story quiet so as not to aid the Russians (since he was pushing for having Trident removed from Scotland, and ultimately no nuclear deterrent). Instead, the Admiral went on TV to push the line that it was a Russian sub that sank the trawler, and no mention of any other issues.
It was a subversive drama overall IMO, especially with the knowing lying of the PTBs as the main final act of the drama with the connivance of politicians and police, but with a bit of a boot into the Russians as well (ie that they actively try and recruit our armed forces and encourage them to sabotage, and even kill - we of course would NEVER EVER DO SUCH A THING).