A stern BBC announcer warned us of disturbing scenes, but we had to sit through lots of standard Missing action - Emily's constant weeping and Tony's constant rage - before getting to the truly disturbing parts.

This final episode opened in a housing estate in snowy Russia. Scratched into the frost on a window pane was the now familiar child's drawing. Outside, a grey figure stamped his way through the snow towards a group of children who scattered at his approach. We were far removed from the quaint, cobbled squares of small town France.

Then we moved to the Hotel l'Eden which, with the discovery of Alain's sobriety coin, is no longer the reassuring place where Tony and Emily could shelter, but is now transformed into a creaking, cavernous old place which may have sheltered Olly's kidnapper.

The group confront Alain, who is now dying in a hospital bed. Tony's fury can't force any truth from him about what happened, so Emily tries, taking his hand and saying, 'I'm begging you'. The truth finally begins to come out. Maybe.

We flashback to 2006, back to that awful moment in the bar where Olly vanished, but this time we go with him, seeing him follow a fox into the bushes and then out onto the road where a drunken Alain knocks him down.

This was the disturbing scene we were warned about - and the most disturbing one of the series. We were blessedly free in this segment from James Nesbitt's angry over-acting and from Frances O'Connor's constant tears. The terrible power of the scene came from the silence as Olly lay in the road under bright lamplight.

But if they were trying to shock us - so it was all because of a fox and some Pernod! - they were wrong, as we all knew Olly was still alive at this point, even as he lay still and bleeding on the ground. He had to be alive because he drew that little figure on the basement wall.

Yes, Olly was indeed still alive, but Alain didn't realise and, in a panic, concealed  the unconscious boy. When a gangster is called to 'get rid' of the body, he does indeed 'get rid' of the little living boy he finds in the basement.

'You're a monster', says Emily and her face breaks into fury and how satisfying it was to see her no longer as a weepy dishrag.

There quickly followed two suicides, and I instantly forgave The Missing for its often meandering, scattered, disparate storytelling. Yet up against the shock of these plot developments was an ambiguous ending. Who was the Russian boy? Has Tony found Olly at last or has he simply lost his mind, condemned to be forever roaming the earth, chapping doors and howling for his son? It seems we'll never know, as Series 2 deals with a new case.

I assume then we just make up our own minds about Tony's sanity, but that's rather a cop-out. Great works of literature can have uncertain endings, like Charlotte Bronte's agonising Villette, but this isn't great art and so it should have done its job and concluded properly. To do otherwise is just frustrating, especially after making us wait throughout eight long episodes.

Despite the rickety ending it has been a good thriller. But that's all it was - a thriller - unlike, say, The Fall which is driven by character, not plot, and lingers demonically in the mind afterwards. Things which depend wholly on plot are too easily dispensed with. Those which depend on character can endlessly unfurl and charm and provoke. That's why The Missing cannot compare to the brilliance of The Fall, which reaches its finale on Thursday and you can bet it won't be a vague one.