Right, that's long enough for you to have seen the episode. If not beware of spoilers, although I won't spoil the big finale. At least not totally...
What a cracking episode.
It starts off as a typical episode of Luther. (Well, typical for Luther but very few other BBC dramas.) Some bad guys kidnap a man and his wife demanding diamonds that they know the man has. The make their point by cutting out the wife's tongue (mercifully off camera). The husband can't tell them where the diamonds are as his wife swallowed them to smuggle them out of the country and he's pretty convinced that the bad guys will cut her open to get them.
How's that for a set-up.
So the husband is released to get the diamonds and goes to the police station looking for Luther's partner Ian Reed. He gets Luther instead and they start looking for the bad guys.
Except Ian Reed it turns out is a dirty cop who is involved with some criminals who in turn are involved with this episode's bad guys. He's off trying to undo the damage, rescue the wife and not get caught.
And everything that can go wrong does, the body count rises and Luther finds out that Reed is involved. Reed goes to visit Zoe and asks her to phone Luther.
And then it all makes sense. All the scenes from the previous episodes where Luther smashes stuff and shouts and throws stuff around. The scene in the first episode where he goes to the ex-wife's house to punch Paul McGann and she calls the police. All of a sudden it all makes sense as Luther says "what have you done?" And you think to yourself "that's a bloody good cliffhanger and I did not see that coming."
I should note that Ruth Wilson is only in one scene of this episode but the preview of the next episode bodes well:
Luther: "Alice, I need you to help me."
Alice: "Where do we start?"