By Karen Fisher

Random House. 366 pp. $24.95

Few writers have the good fortune to find an idea in their closet -- a notebook, or a cache of letters -- but that is what happened to Karen Fisher, and it forms the basis of her debut novel, A Sudden Country. From pieces of a journal written by an 11-year-old, Fisher weaves the story about Emma Ruth Ross and her family, the Mitchells, on the Oregon trail. "On April 15, 1847 we started from near Cedar Rapids Iowa," little Emma wrote. "After traveling six miles we were detained by a snow storm. We made a company of 42 wagons. . . . Past Missouri River was a violent storm the wind caught one of our wagons and overturned it, a man who was our driver killed beneath it. He had been with us 3 days."

An estimated 2,000 people crossed the Missouri that April to journey on the Oregon Trail, in wagons drawn by oxen or horses or even on foot for great stretches of time, and it is in Fisher's fictionalized account of the Mitchells' journey, and, in particular, the story of Emma's mother, Lucy Mitchell, that A Sudden Country is at its best.

Lucy is finely portrayed as the reluctant traveler, content with her Iowa home and all its civilities: the china, the good furniture, the orderliness of society. But she's also a woman of her times and, as a result, obedient to her husband, "the true grandnephew of Daniel Boone, which he mentioned on all appropriate occasions. His parents had toiled. . . . They'd roughed it on the frontier and come out the better side. So when windows needed washing, . . . [they] must appreciate that there were such windows to be washed, not mere flaps of oilcloth. . . . And now they must appreciate this most of all: the chance to go to Oregon, and suffer for themselves those hardships that had made his parents such outstanding characters. The irony seemed to escape him."

What follows is Lucy's immersion in another lifestyle and landscape in which she discovers freedom and, curiously, the deeper attachments made available by that freedom: "The girls made their beds outside on the grass. . . . For all the dwindling down of their lives, their wearing-out shoes, poor dresses, no rooms to live in, and so much left behind, they did seem as happy as ever, sharing songs and stories. Making themselves new out here, and strong, and she thought she loved them even more than she had before they'd left. Though she'd noticed them much less." Here is where Fisher's work reads truest: the finely rendered scenes of ordinary people at the end of resources and in untenable circumstances. There are treacherous river crossings and sudden encounters with an Indian nation growing weary of the newcomers' trespass.

Less successful is the other half of this book -- a parallel story of a mountain man, James MacLaren, who, after smallpox kills his half-Nez Perce children, sets out to find the Indian wife who deserted him. In his wanderings, he is drawn into the Mitchells' journey and becomes a member of their party. In this telling the narration strains, and in Lucy's romance with MacLaren the novel takes the lower road. During an early meeting, after an evening of campfire tales, Lucy shifts from the sensible wife and mother she's been, and her story is reduced to a pulp romance: "An exalted sanity overcame her," Fisher writes. "He was speaking or not speaking, and she heard it; she glowed, she breathed, afraid this sense would leave her." One evening with this stranger in his leathers, his long, unkempt hair, and campfire tales and she's ready to risk it all -- her family, her reputation, and children? "Nothing that had once obliged her seemed in any way important. Some other path had opened now; she felt a strength, an awe, to see in this man's face a life of such great scope and deprivation." Her consequent swooning feels unearned, unbelievable, more in keeping with a bodice-ripper than an epic journey.

Sadly, this story line is a detraction from vastly more interesting aspects of the novel: an ordinary woman venturing into unknown country, a man caught between the world of the natives and the world of the invaders, a frank exploration of the prejudice and anger that were so much a part of this country's making. To give the author her due, although she chooses a too familiar model of mountain-man romance, she takes it to a less predictable ending. Those who survive this journey seem, if not transcendent, significantly altered and at home with who they become. *

Claire Davis is the author of two novels, "Winter Range" and "Season of the Snake," and a forthcoming collection of short stories, "Labors of the Heart."