By Nicholas Evans

Putnam. 403 pp. $26.95

It is early morning in Montana, winter nearing its end but still cold enough to put a hard glaze on the mountain snow. A father and son, skiing on the icy slope, lose control and tumble into a crater. At the bottom of the hole opened by their fall lies the body of a young woman, blond hair and white face floating ethereal and innocent in a casing of ice.

After this dramatic and eerie beginning, we expect to be led into a web of mystery by an engaging sheriff named Charlie Riggs, but "The Divide" suddenly shifts directions. The body is identified almost immediately as Abbie Cooper, who was not innocent at all. With a man called Rolf, she had been making war on the land's despoilers through the Earth Liberation Front -- or ELF, as Abbie often abbreviated it with spray paint. After a year at the University of Montana watching mine owners tear apart the mountains, she had moved from marching in protest parades dressed like a genetically engineered vegetable to burning down buildings.

The current focus on terrorism may have led Nicholas Evans to incorporate it into this plot, but the drama he is really writing about is how people survive when something that could never happen does. Abbie's journey from upper-class golden girl to terrorist on the run is not the cause of her parents' divorce but her response to it.

Sarah Cooper and her two children had lived in confidence that the unity of their family was forever. But suddenly Sarah's husband, Ben, simply leaves. This is hard on the family but less hard on the reader, who will probably find Ben irritatingly weak, both as a husband and a character. Although he has occasional flashes of humor and insight, mainly he epitomizes the description a friend of Abbie's gives of the opposite sex: "Men. . . . They cannot help themselves. They don't know who they are or why God made them."

Surprisingly, given the drama surrounding Abbie, the plot is carried by Sarah and her son, Josh, a gawky, inept teenager who, in the period following his father's departure and his sister's disappearance into the eco-underworld, catches up with himself and turns out to be, in his own bumbling way, a man you can count on. He's also a man who shares the weary, cynical thoughts of his age group in a way that gives life to this book. For instance, on his first solo vacation with his father, a trip to Provincetown, Ben slings an arm across Josh's shoulders, and Josh, imagining people staring at them, longs for a placard that says, "Listen, it's my dad, okay?"

Evans is good at the detail that brings a place or a person to life -- the overly respectful hush of the funeral home's reception area where "in the far corner, a muted TV was entertaining a coffee table and a pair of empty couches in blue velour," or Sarah's father working out on his running machine, determined to control his aging body as he has always controlled his world. "His eyes were fixed on the mirrored image of himself on the wall in front of him, though why, she had no idea, for it wasn't anyone's idea of a pretty sight."

Evans is less good at fitting his stories into a fast-paced narrative. The book would have benefited from less attention to the couple's marital history and fewer peripheral characters, who seem to be thrown in merely to be confidants to the principals. But he writes well enough to be forgiven the times when his novel ambles, and he reminds us that the destruction of all that's familiar -- whether by human hands or by nature -- eventually ceases to be the story. What remains is how people survive.