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There's always been something rather alien about Kevin Spacey, so it was only a matter of time before he played one. That's if he actually is, mind you, because Iain Softley's elegant yet frustrating fable plays on uncertainty. Is Spacey's Prot (pronounced to rhyme with goat) a benign visitor from the planet K-PAX, or is he just some nutcase who's concocted an extraterrestrial alter ego in order to protect himself from the ravages of the outside world?

That's the dilemma facing Dr Mark Powell (Jeff Bridges), a fiercely rational Manhattan shrink with no time for his patients' delusions (""Who is it this time, - Jesus Christ or Joan of Arc?" he quips when handed Prot's file.) This guy, though, is something else. His stories of a planet with two suns and seven purple moons sure as hell sound like the babblings of a madman - - but how come he can see ultraviolet light? What's with his detailed knowledge of planetary alignments? And would a human really eat enough bananas to feed a barrel-load of monkeys, skins and all?

But if Dr Powell really wants to solve the human/alien conundrum, maybe he should just rent Spacey's previous movies to explain his patient's peculiar behaviour. For this is not so much a visitor from outer Spacey as Kev's Greatest Hits. For Prot's sarcasm look to American Beauty's Lester Burnham; his apparent omniscience is taken from Se7en's John Doe; and the gift of the gab comes courtesy of The Usual Suspects' Verbal Kint.

What's more, the feeling of déjà vu is reinforced by the Cuckoo's Nest of loonies Prot encounters at Powell's hospital, the Equus-style uncovering of Prot's hidden trauma and the thoroughly clichéd way workaholic Powell neglects his wife (Mary McCormack) and daughters. Throw in the fact that Bridges himself played a not dissimilar alien - - if that's what Spacey is - - in Starman, and there's a real sense that K-PAX is sewn together from other, superior movies.

Visually, though, Softley's film is its own animal. Taking its cue from Prot's assertion that he has journeyed to earth on a beam of light, cinematographer John Mathieson intersperses the claustrophobic dialogues between Spacey and the excellent Bridges with numerous shots of sunlight refracted through trees, prisms and water. The result is visually arresting, reflecting the movie's themes of perception and - at the risk of sounding bucket-fillingly mawkish - the beauty of both humanity and nature. Likewise, the first half impresses, with layer upon layer of intrigue applied until you're bursting to get to the heart of the mystery.

It's just unfortunate that a premise so stuffed with potential then fizzles out and Softley's sci-fi parable remains resolutely earthbound just at the point it should really soar.

Jeff Bridges wins this battle of acting heavyweights, battering Spacey's mannered turn. Entertaining, good-looking and thought-provoking sci-fi, if too derivative to linger in the memory long.

The Total Film team are made up of the finest minds in all of film journalism. They are: Editor Jane Crowther, Deputy Editor Matt Maytum, Reviews Ed Matthew Leyland, News Editor Jordan Farley, and Online Editor Emily Murray. Expect exclusive news, reviews, features, and more from the team behind the smarter movie magazine.