By Mary Wesley

Viking. 364 pp. $18.95

Mary Wesley's seventh novel is not her first to be published in this country, but it seems to be the one for which what publishers call a "breakthrough" is hoped if not predicted. "A Sensible Life" quickly became a bestseller in Britain, bringing its septuagenarian author happy if belated reward, so it's natural that expectations of a similar performance on this side of the Atlantic have been raised; unfortunately, if they are fulfilled it will be a triumph not of the wit for which so many British writers are known but of the cliched contrivance that is the hallmark of American commercial fiction.

"A Sensible Life" is a few notches above Judith Krantz, but not so many as you'd expect from a writer who comes so highly recommended as Wesley does. To be sure the prose is smoother and more agreeable than is to be encountered in most American books -- the same, alas, can be said of almost any book by almost any British writer -- but as compliments go, that isn't much. Otherwise, "A Sensible Life" is endlessly talky, all the more endlessly since so little of the talk is interesting, and packed with enough improbable coincidence to embarrass even Sidney Sheldon or Robert Ludlum.

The novel begins in the spring of 1926, a time when "middle-class English families took their young to Brittany for the Easter holidays" so that they could have "a glimpse of foreign soil, see some sights, learn a few words of French." In this particular spring a few families have gathered at St. Milo and, after scores of pages of preliminaries, have held a picnic:

"People recollecting the picnic in later years remembered the surprises and how each surprise had surprised. The food, the wine, Flora's langoustes, the jokes, Mabs and Tashie singing, Felix's gramophone, Freddy and Ian dancing, the moonrise, and finally the rockets and Catherine wheels. We forgot we had to go back to school, they said. We forgot the General Strike, and of course next day it rained. My God, how it rained: we piled into the vedettes in pouring rain and that curious child, what was her name, can you remember, stood on the quay without a mackintosh weeping. She can't have been weeping for her parents; they had no time for her, they were so obsessed with one another it could almost be said they neglected her; some people actually said so. It was quite odd to see a child cry like that."

The "curious child" is Flora Trevelyan, who is 12 years old and has made "the terrible discovery that she was in love with three people at the same time." They are, more or less in order of her adolescent passion for them, Felix Habening, Hubert Wyndeatt-Whyte (also known as "Blanco") and Cosmo Leigh. If they sound like cardboard gentlemen in a bodice-ripper, well, that's what they are, even if "A Sensible Life" isn't. But Flora's relationships with them keep the novel huffing and puffing during the 250 pages through which the reader must wade if for some reason he is curious about its conclusion.

For most of those pages "A Sensible Life" belongs to a genre that seems at last to have exhausted itself: the Novel of War and Courage in which at some point -- you can see it coming from the book's first sentence -- someone asks, "Is this Hitler chap the sort of fellow we will get to hear of?" and in which the voice of Vera Lynn can be heard in the background. Whether in fiction or nonfiction, it's the same old story: the privileged young men and women in their Finzi-Contini gardens, sucked toward war and destruction even as they dance the night away, saying their endless farewells and writing their jolly letters and postcards.

For those who lived through them those years and those experiences were all too real, and for a long time they've been the raw material for a great deal of fiction and nonfiction, some of it powerfully moving. But "A Sensible Life" borders on unwitting parody of the genre, what with an excess of heavy-breathing dialogue and numerous twists of plot that pass from the coincidental to the preposterous.

"A Sensible Life" is offered to us as a work of sophistication and wit, but it's really a potboiler, if rather more intelligent than most such. If it were at least entertaining its deficiencies could be excused, but it isn't.