BE afraid, be very afraid. Signs (12A), the third film from Oscar-nominated writer-director M Night Shyamalan (The Sixth Sense, Unbreakable) is a masterful exercise in sustained tension.

Signs focuses on an ordinary family caught up in extraordinary circumstances, which rapidly spiral out of their control.

The picture demands a Herculean leap of faith from the audience. But if you're willing to jump with Shyamalan, you'll be rewarded with a psychological rollercoaster ride that chills to the marrow.

In the close-knit community of Bucks County, Pennsylvania, preacher-turned-farmer Mel Gibson has raised a family almost single-handedly.

His beloved wife, played by Patricia Kalember, died in a freak road accident, and Graham has never been able to forgive God for the injustice.

Nevertheless, he struggles on, if only to keep a roof above the heads of his brother Joaquin Phoenix, and children Rorty Culkin and Abigail Breslin.

Overnight Gibson becomes a local celebrity when intricate 500ft crop circles appear in his cornfield.

At first, the farmer declares the patterns to be an elaborate hoax, perpetuated by a group of rowdy locals.

However, the deeper Gibson digs, the more outlandish and disturbing the truth becomes.

Signs is one of those rare films which gets beneath your skin and refuses to go away, energised by a series of shocks and scares that grows naturally from the simple premise.

Shyamalan understands there's nothing more terrifying than the unknown, and he's careful to keep the on-screen violence and gore to a minimum.

Unlike the director's previous films, there's no last-minute twist as such in Signs, but the script conceals a number of ingenious surprises, revealed in the gripping final ten minutes. Instead of visual thrills there is just an impending sense of dread, a feeling that out there in the corn something is lurking.

Gibson delivers a commanding performance as a fallen priest wrestling with his faith, and he bonds well with Phoenix, Culkin and Breslin, each of whom has their part to play in the unfolding drama.

The children are particularly excellent and the director recognises their naturalness accentuates the tension and fear of the film's most frightening portions in much the same way Steven Spielberg used his young actors in Close Encounters of the Third Kind.

Indeed there are many parallels between the two films. Gibson's dogged pursuit of the truth, no matter how little he understands of it, is reminiscent of Richard Dreyfuss' desperate quest for the meaning behind alien contact in Close Encounters.

But unlike Spielberg's masterpiece, there are a few glaring flaws in the Sign's logic. And once the finale reveals what's really happening, the film's premise becomes even more puzzling.