First published in France two years ago and now translated into English, Chantal Thomas' novel "The Exchange of Princesses" tells the melancholy tale of two girls put forward as diplomatic pawns in the first quarter of the 18th century.

The scheme, the brainchild of Philip d'Orleans, Regent of France during the minority of Louis XV, was to betroth the 11-year-old king to the Infanta Mariana Victoria, 3-year-old daughter of Philip V, King of Spain, and further, to betroth his own 12-year-old daughter, Louise Élisabeth, to Philip's 14-year-old son and heir, Luis. To this end, the two girls were sent off on nightmare journeys across the primitive roads of their native lands and exchanged in January 1722 on an island in the middle of the Bidasoa River, which marks the border between France and Spain.

Louise Élisabeth, neglected and undereducated, is a boor, gorging herself, belching and vomiting willy-nilly. In fact, she's a little crazy, with a penchant for throwing off her clothes. She is treated abominably by the Spanish royal household when she doesn't bloom under their efforts to please her with such Inquisitional delights as the spectacle of heretics being burned alive.

The only people who like her are a few ladies in waiting, with whom she indulges in obscure pleasures, and Luis. He, however, is a feeble creature who disgusts her and she "gives herself over without resistance to her dark moods as though guided by a compass of despair."

As for her counterpart, the Infanta: We see a sweet, precocious, eager-to-please little girl doted upon by the French people and by those around her — except for her intended husband, Louis, who feels an immediate animus toward her. In time, the child's protectors die, she wilts under Louis' disdain and becomes peevish, wretched and despised by the public.

An air of doom, death and decay hangs over both courts, but that will do for now. The novel, which incorporates some telling contemporary letters and reports, continues on to history's implacable denouement.

Thomas pursues the story in the present tense in a detached, oddly benign manner, somewhat in the style of the natural historian. Indeed, I could not help thinking of the great French etymologist Jean-Henri Fabre and his affable observations in "Social Life in the Insect World." But, of course, here the objects of scrutiny are human beings and Thomas' impassiveness has an intended ironical effect, artfully accentuating the coldbloodedness of it all.

Katherine A. Powers reviews widely and is editor of "Suitable Accommodations: The Letters of J.F. Powers, 1942-1963."