Those of you who have been living in a forest hut for the last year and haven't got around to seeing the Coen brothers' No Country For Old Men should stop reading here.

This week my subject is film endings - in particular the frustratingly brilliant climax to their adaptation of Cormac McCarthy's skewed lament for the American West - which continues to give rise to passionate debate almost a year after its Cannes 2007 debut.

No Country For Old Men has what most people, even its most fervent supporters, would recognise as an unsatisfying ending, at least by the standards of classic narrative cinema.

Its apparent hero, Llewelyn Moss (Josh Brolin) is your typical ordinary guy - good but weak, tough but decent - and we accept the script's clear invitation to invest our sympathy in him.

But well before the credits roll, he's dead - killed, it seems, not by the real baddie, Javier Bardem's dream-hauntingly nasty Anton Chigurh, but by some Mexican hitmen that have only just entered the story.

Chigurh makes up for this uncharacteristic lapse by murdering Moss's entirely innocent wife. Soon after, he is involved in a car accident - but this is too random, in ethical terms, to function as payback. Plus, he walks away from it, more or less intact.

Instead of an indestructible hero, we get an indestructible villain.

There's only one cinematic genre in which villains are indestructible - so it appears that what began as a thriller has become some sort of metaphysical horror movie (metaphysical because through the terrifying Chigurh we experience what one horror theorist refers to as 'the immanent fears of mankind', though without any of the obvious satisfactions of seeing expendable characters being cut into small pieces).

But that's not quite right either. Largely thanks to the two strange scenes of actionless talk that bookend the final Chigurh sequence, what the film feels like, in the end, is an elegy - for the passing of the morally dependable American frontier lifestyle represented by Sheriff Ed Tom Bell (Tommy Lee Jones).

Elegy is a poetic genre; the Coen brothers give it dramatic impact by disguising it initially as something else (in McCarthy's novel the melancholic lament for a lost age is far more pervasive). We're made to care about a thriller plot and then made to realise that our caring about it is wrong, or irrelevant.

In other words, if the ending of No Country For Old Men works, it works because we've been slapped in the face (A History Of Violence did something similar, but it managed to eat more of its shoot-'em-up cake at the same time).

And the critical reaction to the ending, which has been running in print and on the blogosphere ever since the film's Cannes 2007 debut, splits between those who resent being treated in this way ('I want to see the focal point of all the drama and tension that's been building in the film,' fumes one blogger) and those who appreciate the therapeutic value of the slap.

Betrayed and exposed

But rationalising it afterwards doesn't stop your cheek from burning at the time. Leaving the Cannes press screening of the film last year, I felt a mixture of frustration and admiration. Seeing it again recently on DVD, the admiration predominated, because this time I was expecting the dramatic betrayal.

It's tempting to argue that films like No Country For Old Men and M(which contains perhaps the most extreme face-slap ending of all time) say something revealing about the psychology of the cinema-going experience; the problem is that what they say is pretty unflattering.

By refusing to cross the finish line, like the Borstal kid at the end of The Loneliness Of The Long Distance Runner, they suggest we're all, deep down, ethical reactionaries craving moral order and resolution (the good rewarded by killing the bad or by the next best thing, a heroic death) even if it goes against what we know about how the world works.

But then, you suddenly realise, this is the real stroke of genius of No Country For Old Men. Rather than show us what old Sheriff Bell thinks, it puts us through the same moral meat grinder; it brings out the narrative conservative in all of us. 'Hell,' we mutter ruefully to each other, 'films ain't what they used to be.'