On Monday I thought Cloverfield was a brand of butter, or a mock-Tudor retirement home in South Wales. The film is actually an old-fashioned monster movie that has been retuned into an unexpected - sometimes unwatchable - sensation. Cloverfield is a reality-TV chiller, written by Drew Goddard, who cut his teeth on Buffy, Alias and Lost, about the most ferocious monster attack on New York since King Kong slipped his chains in 1933. It is also the first horror film that has given me motion sickness.

An hour in I started to sweat. I couldn't look at the grim stroboscopic lighting effects in the final reel, and I nearly threw up trying to make sense of the increasingly chaotic and frightening scenes of the gripping climax. On a giant cinema screen like that of the Empire in Leicester Square the discomfort is rude and disorientating.

Matt Reeves's apocalyptic thriller is a genuine scare. The film begins with a stuttering romance between a young city hunk (Michael Stahl-David), and his shapely squeeze (Odette Yustman). It then plummets into an appalling piece of chaos when an earthquake buries plans for sex in heaps of dust and rubble.

The real stroke of genius is the visceral sense of increasing panic. There is no warning of just how demonic this flesh-eating monster actually is when it is glimpsed by drunks from the balcony of the hero's flat.

The film is recorded like a diary piece on Stahl-David's camcorder. The luckless cameraman is a fabulously useless jock called Hud (T.J. Miller). It is Hud's precarious forefinger on the “record” button that makes the footage of this film such a stop-and-start marvel.

The horror begins with tremors in central Manhattan. The expensive and jaw-dropping stunts that are carved into this cheap camera are extraordinary. This is horror unplugged. A monster is trashing New York - God knows why - but it's one of those films where you feel obliged to suspend every ounce of disbelief.

The devastation looks like a fireworks display. Skyscrapers and bridges are shredded. The torpedo might of the US Air Force merely eggs the monster on. The severed head of the Statue of Liberty is tossed down the street. The young heroes seek refuge in underground stations crawling with spidery, flesh-eating aliens. This is almost a biblical interpretation of 9/11.

The panic is exquisite. The unnerving novelty about Cloverfield is the quality of shock. Fantastic.

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