In 'The Passage,' a military killing machine pursues humankind.

It's hard to say exactly what Cronin intended with this book. It's structured with grace and intelligence, but hokey in small ways throughout. Some elements of the story are genuinely, successfully scary, but others - most notably a few cardboard-flat characters - are limp.

The book's premise, in a nutshell: A group of biochemists (impressive ones from Harvard and Columbia) go to the jungles of South America looking for a virus that could unlock the secret of eternal life, but soon find out they're unwitting participants in a military experiment that is trying to build an indestructible human killing machine.

You won't be surprised when, within the first 25 pages, that little plan takes a bad turn. This first chunk of the story - a full third of the book - makes for fine reading. It's taut and tense and set, cleverly, just a few years in the future, which makes it feel realistic and, with references to recent terrorist attacks in different U.S. cities, queasy-making at the same time.

Trading in wartime unease and a general sense of mistrust of the larger forces at work in our lives, Cronin has set the stage for a fairly believable end of the world. The government conducts its creepy, top-secret testing on a handful of human subjects, infecting them with a virus that turns them into soulless blood-drinking predators. (You know, vampires?)

At the center of all this is a little girl named Amy, one of the test subjects who, with her stoic sadness and wisdom, is easily the most compelling person we meet in these pages.

Wolgast, the FBI agent who was hired to kidnap her, is a sympathetic figure, too, and he forms an attachment to the girl that feels desperately, touchingly parental.

When the virus spreads to the world outside the underground bunker, it moves fast, and within months, the population of America is decimated.

We learn about this in bits and pieces from the perspective of Wolgast, who's holed up and has very little idea of what's going on; this is still appealing, exciting horror-story stuff.

What remains after the outbreak are millions of these nasty vampire things, which, by the way, glow green like a chemical spill and live forever, unless you shoot them square in the chest.

Cronin's vampires are interesting because they're just about the polar opposite of the debonair European dude we've come to associate with the myth. Instead Cronin goes for a wild American frontiersmanship in all its gritty, likable glory. No erotically charged Victoriana here; much of this tale takes place in the desert of the West.

Once the story gets rolling, it feels less like a vampire story and more like a zombie apocalypse. We've got abandoned, blasted-out cities, some ragtag survivors, and a rocket launcher or two. All of which sounds awesome. And it almost is. Then we jump forward 100 years, to a settlement of barricaded survivors in California who are still fighting off the "virals." For all they (and we) know, they're just about the last humans alive. They use blinding lights around the colony's perimeter to keep the vampires at bay, but the batteries are rapidly running out of juice.

Unfortunately, encapsulating The Passage this way makes it sound more exciting than it feels in the reading. Throughout the entire novel Cronin switches perspective from person to person, usually for only a few pages at a time, which is no small feat. This has the effect of subtly suggesting the interconnectedness of humankind that is an important theme for Cronin; he keeps circling it, always coming back to the ideas of family, children and the future.

But it also makes it hard for him, and us, to touch down meaningfully on any one person. Where Wolgast and Amy are pretty fully realized, many of the other characters (and in 766 pages, there are many) are too broadly drawn. They often feel flat and interchangeable, and their relationships to each other aren't interesting. One of the inhabitants of the First Colony, a minor character, is differentiated from the other men by almost nothing other than his beard, which ought to strike us readers as a little silly. What are we to imagine the other survivalist men doing, shaving in front of the mirror every morning?

The Passage has plenty of building-exploding excitement, and a few interesting ideas besides. But it would have done better to make use of more literary stuff, to give us people we can care about, who have emotions we can really feel.

You need that, not just for good fiction but for a good scare, too. Just ask Stephen King.

A new, undead peril stalks us

Justin Cronin:

"The Passage"

7:30 p.m. Thursday, Central Library of the Free Library of Philadelphia, 1901 Vine St. No ticket required. Information: 215-567-4341 or www.freelibrary.org.