Take one less than endearing couple and a plot about shampoo and you've got Duplicity.

Julia Roberts and Clive Owen star in Duplicity.
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There's a scene about a third of the way through Duplicity where, helpfully for your less than assiduous reviewer, the characters spell out the film's central failing. "Is that what makes this worth it?" Julia Roberts wonders at the end of a three-day John-and-Yoko love-in in Rome. "Is that it? That I know you're thinking exactly what I'm thinking?"

And that, disappointingly, is indeed more or less it. Duplicity revolves around the interplay between two people who are perpetually double and triple-crossing each other. Sometime it is for a purpose, sometimes just for kicks, but as each deceitful layer is, in turn, revealed as part of a bigger game, their steady unpeeling leaves the question at the film's heart: why would we care about these characters or their fate?

Ray (Clive Owen) and Claire (Julia Roberts) are the scheming spooks who meet, double-cross each other, meet again, double-cross each other again, fall in love, decide that the only way they can be together is to quit their secret-service jobs, double-cross each other yet again, and go into commercial espionage. Whereupon, naturally, they double-cross each other several more times, because you need to keep some spice in a relationship.

Having toyed with trading secrets in the heady world of frozen pizzas, they settle instead on the high-wire industry that is shampoo, cooking up a plan to plant her inside a giant cosmetics corporation as a mole, while he works for its rival. Together the pair will extract the secret formula, and sell it on to the highest bidder. The film tries to give the shampoo some drumroll. It is no ordinary shampoo, with provitamins, magic powers, the works. We are repeatedly reminded how "beyond huge" it will be and Claire evidently believes it a sufficient vantage point from which to sneer at Ray's proposed pizza-based skulduggery; the viewer may be less sure.

With better writing, or some actual action - there is precious little beyond some pedestrian snooping and light shenanigans in a casino hotel - the innate dullness of a product as prosaic as shampoo need not have been insurmountable. Previous attempts at the spy-couple genre (True Lies, Mr & Mrs Smith) laced the formula with plenty of bangs. Instead, Duplicity gives us poker faces; a prettily blank couple regarding each other with prettily blank mutual suspicion, over and over. She's playing him! Again! Who'd have thought?

It is perhaps telling that this is only Tony Gilroy's second shot as a writer-director, on the back of his screenplays for the far superior Bourne trilogy. Those films were notable for their thrillingly crunchy fights and chases, but not for crackling dialogue; Gilroy would perhaps have been better to expand his oeuvre with something more action-based. Still, the script is not entirely without charm. The opening scene, an ambassador's reception in 2003 Dubai (with a visual cue straight from the cheat sheet: half-built. One wonders how long that tag will stick), has Ray hitting Claire with some splendidly cheesy woo, prevailing and then, naturally, getting double-crossed for his pains.

There's enjoyable jousting, too, when Claire fakes failing to recognise Ray, years after that Dubai encounter. Unfortunately, Gilroy was evidently so pleased with this ribaldry that he makes a motif of the scene, playing it out thrice more over the next two hours. The second time out, it's an amusing echo; by the fourth it only drives home how few other high points are on offer here. Neither is Gilroy an incompetent director. There are some lovely visual flourishes - the slo-mo title-sequence fight between the preposterous rival chief executives - Tom Wilkinson (The Full Monty) and especially Paul Giamatti (Sideways) - is a joy, as is most of the limited time that Giamatti's wonderfully dyspeptic snarler is on screen. The film's structure, intercutting between the present and a chronology of Claire and Ray's encounters, is hardly original but works neatly enough, lending it a decently watchable pace that's all too rare.

The wry denouement is fun, if predictable, but in the end Duplicity struggles to provide much to compensate for the twin deficiencies of its unlikeable leads, and a mission that is, if anything, even less engrossing than they are. One cannot help but feel, in the end, that it is the viewer who has really been double-crossed.