CONSPIRACY THEORY Director: Richard Donner Starring: Mel Gibson, Julia Roberts, Patrick Stewart (15)

There is a sequence about 30 minutes into this initially promising thriller which suggests that there may be something more to what we're seeing than meets the eye. As a paranoid New York cabbie and the brains behind a conspiracy theory newsletter, Mel Gibson seems at first like your usual opinionated, motor-mouthed taxi-driver. The film opens with him detailing various conspiracy theories to hapless passengers who simply stare in amazement at his ability to talk - and talk drivel - without taking a breath.

And then, out of the blue, he is abducted and subjected to a protracted torture sequence which the director, Richard Donner, a man who's known for his visual daring, chooses to relate to us mostly through the victim's eyes. It's not a radical move for a Hollywood film to take its hero's perspective, but it's unusual enough to be noteworthy when the camera gets inside the suffering, and the hallucinations, which that hero is experiencing. In this case, Donner employs stroboscopic lighting, coloured filters and, strangest of all, cartoon excerpts to give some indication of the loose wiring inside Gibson's head. An action movie in which the male lead resides in cloud cuckooland? Does this not amount to a revolutionary act?

No, sadly. The sequence is a one-off. Until then, Donner has been building the tension with great care. Gibson's emotionally fraught encounters with an attorney (Julia Roberts), whom he believes shares his predilection for stories of espionage and duplicity, are nicely handled and beautifully capture the waves of guilt and unease which arise out of a relationship which only one person is struggling to maintain. Roberts is best in the scenes where she tries to keep Gibson at arm's length - a final attempt at uniting them in romance simply doesn't wash. You can believe that this woman would write him postcards, bake him angel cake, maybe even call him up once in a while. But fall into his arms? Not in this life.

It turns out that Gibson really is onto something, having once been part of some shady dealing which left his brain scrambled and prone to only occasional flashes of earthly logic. His tormentor is the icy Dr Jonas (Patrick Stewart), a character whose reserves of menace are completely expended before his first scene is through. This leave Conspiracy Theory with a definite absence of tension: we know the villains, and we know their motives. All that there is left for Gibson and Roberts to do is to evade and eventually overpower them.

Any surprises the film has to offer are buried in Gibson's restless performance. But too often Gibson is encouraged to play up the bumbling, goofy side of his character, so that he stops being troubled and disturbed, and starts being a lovable kook. This dissipates most of the tension created by the claustrophobic production design and Carter Burwell's haunting score, and tips the film toward being a caper comedy, a genre in which it simply isn't equipped to survive.