As expected, the penultimate part of BBC One’s One of Us really kicks things up a gear.

Several times before the close of Episode 3 I found myself thinking that this, surely, must be the end: the police arriving at the funeral!  Both families taken into custody! Rob’s confession!

When it comes, the cliffhanger is something else again, and seemingly opening another can of worms, all of which promises a highly-charged final episode next week.

Back to this week, though, and along the way we clear up at least a few mysteries.  For one thing, Moira and Alastair have been having an affair.  It’s not obvious just how long it’s been going on, but clearly long enough for him to have written her that drawer full of letters which she burned last week.

We also discover that husband Bill has Parkinson’s disease, something he has kept from Moira but shared with the children, who have been tested too.  This conveniently allows son Jamie to spill the beans in a fit of petulance outside the police station.  Although he’s a bit irritating, it’s maybe a bit hard to criticise him for telling the police about his parents having covered up a murder.

Be that as it may, I don’t think either of these ‘resolutions’ are simply the tidying up of red-herring subplots.  The blood tests suggest some tangled business over parentage – perhaps the murdered Grace has turned out not to be Bill’s after all (which I guess brings us back to the question of just how long Moira and Alastair have been at it).

The show’s other father, Ade Edmondson’s Peter Elliot, is also involved in upping the drama.  Having set off on the long journey from London to Scotland last week, I was expecting a frosty reunion with the family – but no, this week sees him parked up in a layby shortly before walking out onto the motorway where the inevitable quickly happens.  It’s a genuine shocker, precisely because we don’t expect it until just a moment before he does it.

Peter leaves a last phone message to daughter Claire, saying “It’s all my fault” – and again I don’t think this is a generic ‘death bed apology’ from a neglectful father, but something much more specific and somehow linked to the double-murder of the newlyweds which started the series.

The home video footage of their wedding has been such a regular presence throughout the episodes it’s been obvious that sooner or later something significant was going to show up.  I thought probably killer Lee would be spotted lurking at the reception, but thankfully the writers’ imaginations are much better than mine.  Leaving the tape running, Claire comes across an angry row between her brother Adam, and Bill…

Which brings us back to where we came in, because the actual end of the episode is Claire confronting Bill over the recording and insisting he tell her exactly what’s been going on.

I’m with Claire, I’d very much like to know what’s going on – and it will be a long seven days until we find out!

Aired at 9pm on Tuesday 6 September 2016 on BBC One.

Buy One of Us on DVD on Amazon here.

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