Before I Go To Sleep turns out tense enough to keep you awake ...

Film Review The mark of a good thriller is usually that you don’t guess the who or the why or the what of it. Or, if you do manage to guess the who, why or what, at least you shouldn't be able to guess the how. But sadly, all that can be undone, if, after the big reveal, your first thought is: "But wait a minute, then why didn’t X and Y happen?

That’s the main issue afflicting Before I Go To Sleep – turning it from a pretty good psychological and neurological thriller into an average one. Well that, and an incredibly maudlin ending.

Nicole Kidman takes on the role of Christine Lucas, lead character in the 2011 debut bestselling novel by SJ Watson. Lucas is an amnesiac woman whose nightly rest erases from her mind all that’s gone on the day before. If you’re thinking we’ve been here before with Guy Pearce, you’d be right, there’s more than a shade of Memento going on here as Christine tries to figure out just how much of what she’s told by the people around her each day is really true.

Chief among these daily diarists for Christine are her husband Ben, lackadaisically played by Colin Firth, and her neuropsychologist, Dr Nash, ably portrayed by Mark Strong. Firth appears a bit like he’s reading aloud a script he only just got into his hands a moment before the camera started rolling, but Strong puts in a decent effort. Everyone is trying to come across as both sinister and sincere at the same time to try to keep the audience guessing, doubtless not an easy task, but it does mean that none of the secondary characters really have much depth to them.

That leaves Kidman to carry the whole thing off and she does it well. Particularly fun are the scenes where Christine pokes and prods at her 40-year-old face, expecting instead to be seeing herself as she looked in her twenties. (The fact that Kidman is rumoured to have done some work on staving off the signs of ageing, and that Christine was 47 years old in the book, add a twist of humour to her predicament – whether she’s in on the joke or not.)

Up until the final moments, Kidman is also adept at keeping sympathy levels high for Christine without degenerating into sentimentality. You really feel for her as she relives all the moments of heartbreak from a whole lifetime each day as if the bad news were fresh and new and all coming at once. Disappointingly, the mawkish ending rather ruins the hard work she’s done by descending into one of those power-of-love-over-everything messes that Hollywood’s so fond of.

However, viewers may not even notice that, as they could be too busy spotting the enormous plot holes in the whole thing. Whether you'd guessed it or not, the thrill of the big reveal is pretty decent and will carry you through the action-packed pay-off.

Slowly, though, you’ll realise that the whole thing is one of those situations that depends on certain key individuals not saying anything about things you’d kind of expect them to talk about – and others behaving in complete contradiction to the character traits you’re supposed to believe they have. And once you know that, the whole thing feels like a bit of a waste of time.

It’s tough to get a psychological thriller right and despite some good ideas and the vast potential of anterograde amnesia, this one doesn’t quite get there. Maybe the book is better … ®