by JULIET WATERS

I once interviewed Ian McEwan, who told me that after every complex, serious book he writes, he usually shifts to what he calls "a vacation work." In the past, this was usually a screenplay. But after a script he wrote for the sequel to The Fly ended up on a shelf somewhere in Hollywood, he went into what he called "a pout" against the film industry. The vacation work he was working on then turned out to be Amsterdam. Sure, it went on to win a Booker, but it sparked a small, shrill protest by devoted fans that this nasty, readable little satire should be the first McEwan book most people would read.

So after his last novel, Atonement, McEwan was on track for an easy read, which Saturday is. The book is short and the plot is simple. A chance encounter in the day of a brilliant, contented London neurosurgeon turns it into a day of horror. Unfortunately - maybe afraid he might win an award and face more angry fans - McEwan has loaded what could have been a compulsively readable short novel with mini meditations on just about every theme he's ever written about in his career.

McEwan is a master storyteller with an unparalled genius for emotional detail and plots designed to efficiently detonate moral complacency. But as a social philosopher, he's become a little too enamoured with the world according to Stephen Pinker. This is a world where lives, character, tastes, moral capacity, sexual preferences, etc., are more often than not dictated by genetic makeup. Humans go along in their normally well-programmed way, unless one of them should rub up against a circumstance or person who's not part of the brain plan... and, voilà: drama.

To the extent that he makes this formula almost awkwardly obvious, Saturday is indeed a vacation work. McEwan has clearly done a tremendous amount of research in creating Dr. Henry Perowne, but too often Saturday reads like a doctor becoming a character in his own compelling case study. Just as you're heading into spine-chilling suspense, the tone shifts to clinical. Fortunately, just as it hits clinical, it shifts back into suspense. And so we have a new genre, weirdly appropriate for our time: the terror essay.

If you go in for the right brain-left brain theory of personality, Perowne is as left as left can be. He respects order, hard work, God in the details and music. He just doesn't get all that right-brain stuff, i.e. literature and poetry. This is unfortunate, given that his father-in-law is a renowned British poet, and it's looking like his daughter is on track to become one too. At least his son is growing up to be a brilliant blues guitarist. If you thought that children with generally happy lives were unlikely to grow up to be important artists, well, McEwan has written a novel to prove you wrong. Everybody in this novel seems to be more of an interesting brain that a complicated human being.

The Saturday in question is in February 2003, and Perowne's ruminations about the impending war on Iraq take on a new dimension when his family becomes the victim of a house invasion. Suddenly, they're uncomfortably similar to an Iraqi family who might happen to find themselves one Saturday on Saddam Hussein's naughty list. One day everyone's living a civilized dignified life, then, in matter of minutes, they're more like animals being tortured by animals.

To reveal much more would spoil a scenario already challenged enough by credibility. But whatever Saturday's problems, McEwan is wired for storytelling. No matter how hard he tries to write a somewhat facile novel of ideas, he just can't do it. It's an improbable story, but a compelling one. One that may occasionally irritate, but probably won't disappoint his readers.

Saturday by Ian Mcewan, Knopf Canada, hc, 288pp, $34