My first fleeting encounter with the work of Robert Charles Wilson was when I found copies of two of his earlier novels piled high near the back of a remainders bookshop. A friend told me Wilson was good, but he didn't convince me to part with my 99p. My second encounter was when I was given a copy of Mysterium to review. Next week I'm going back to that remainders shop in the hope that I'm not too late.

Although sf gets a mention in the blurb, the use of phrases like 'over the edge of terror' and 'nightmarish dislocation' and the obligatory predominantly black cover seem to imply NEL are packaging this book as a horror novel. In reality it's more Twilight Zone sf, an intense culture-shock thriller that tackles, amongst other things, cosmology and gnostic religion. The basic premise of Mysterium is nothing strikingly original, there's no continual dripfeed of startling ideas: all we have is a single catastrophic event, followed by the inexorable exploration of its consequences and how a group of people cope.

The novel opens with the discovery of a strange artefact in a dig in Turkey in the late 1990s. When three archaeologists subsequently die of radiation sickness the US army move in and take the object into their care. Later, a research laboratory is established near the remote town of Two Rivers, Michigan, and a top physicist recruited to study the artefact.

Then, one spring day, the town and the laboratory vanish. All that remains is an uninhabited wilderness and a series of forest fires caused by severed power lines. And a wandering lunatic who gives as his address a nonexistent Boston street and accuses those who find him of being Mohammedans or worse. Yes, when stripped to the essentials, Mysterium is a fairly straightforward story of parallel worlds.

Two Rivers and its surrounding countryside have, to borrow one character's phrase, 'somehow travelled while standing still' into a world governed by a brutal regime founded on the teachings of the Christian Gnostic Church. Before long the town is occupied by the region's armed forces - curfews and rationing are imposed while the population's fate is decided. Those who challenge this new rule are executed and their bodies put on display to deter others.

Apart from one brief lecture, Wilson avoids the common trap of feeling he has to explain in great detail how his parallel world differs from our own: the outside world is only explored in as much as it affects the beleaguered citizens of Two Rivers. It gradually emerges that it's going to have one pretty powerful effect on them: the religious authorities are concerned that Two Rivers might be a source of ideological contamination. They decide to wipe out that risk.

If sheer originality of ideas is your main criterion, there's nothing particularly special about Mysterium: there are any number of parallel world stories to choose from. But Wilson's unfolding of the consequences of that one idea is beautifully constructed. The characters - each with a distinct individual voice - are carefully portrayed in a progressively deepening way that means you're learning new things about them all the way through.

Even right at the end, the precise explanation of what has happened is left just a little ambiguous - the protagonists are left to understand it as best they can, as is the reader. What you do know is what a thoroughly rewarding experience it has been to reach that point.

Elsewhere in infinity plus:

• nonfiction - more reviews of Robert Charles Wilson's work.

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