The Faithless Priest and the Obsessed Harlot : POISON, <i> By Kathryn Harrison (Random House: $23; 317 pp.)</i>

In 1679 Marie Louise de Bourbon, the niece of Louis XIV, the Sun King, married Carlos II, the last of the Spanish Hapsburgs, in the village of Quintanapalla, Spain. She and Carlos were both 18. The princess was tall, beautiful and vivacious, fond of frolics and horseback riding; the king was short, ugly and gloomy, given to paranoia and superstitions and so many everlasting illnesses that he confined his food to bowls of breast milk supplied by a platoon of healthy wet nurses.

With Carlos’ infirmities and probable impotence or sexual ineptitude, it is not surprising that the regents were childless during the 10 years of their marriage, but the fault was laid on the foreign wife who became hugely disliked by a people who regularly fought wars with France. In the afterword to her novel, Kathryn Harrison notes that on Feb. 8, 1689, Maria Luisa, as she was called in Spain, fell from a horse and was put to bed. “At five o’clock on the morning of February 10, she awoke feeling suffocated and suffering a severe gastrointestinal upset. Her condition deteriorated rapidly throughout that day and the next, and she died on the morning of February 12. While it was never proved that the queen was poisoned, most historians assume that she was.”

“Poison” is a fantasy on the Queen’s life and death told by the fictional Francisca Luarca, the daughter of a failed Castilian silk grower, as she is held in an underground prison during the Inquisition. King Carlos had made an official statement from the royal balcony in the Plaza Mayor: “ ‘The failure of Queen Maria Luisa to get with child,’ he said, ‘is due to sorcery.’ ” Within a day 17 witches were found in the royal residence, and all persons who’d been employed in the palace from the year of Carlos’s birth until the present were investigated. Since Francisca’s gracious and bountiful mother had been one of the child king’s wet nurses and was now dead, the Luarca family fell under great suspicion and the Inquisition found out that Francisca often wandered far afield, which was at best unseemly, that she’d been taught to read for some possibly nefarious purpose and that her teacher was a faithless priest, Alvaro Gajardo, by whom she was pregnant and with whom she was obsessively in love. Either she was a witch, then, or a fool.

“Alvaro’s fate was certain: he would be tortured; whatever confession he made would be recorded. For the sake of his soul, he would be pressed to implicate whatever other sinners he could. But he would not betray me, he would do what he could to save me and our child. After they had as much as they needed, or as much as they could get, the Holy Office would excommunicate him, and the Church would then abandon Alvaro to secular justice. The Church sheds no blood, not even that of denounced heretics and seducers. Spain, however, would take her due.”

Francisca’s fate is less furious but no less painful. She could not be hanged or tortured while she was carrying a child, and she was thought to have been abused and led astray by a priest “so the Church could hardly punish me as it might any other harlot.” She is let free, then, to mother her son, to be feared and hated by townspeople, to grieve for Alvaro, to seek healing miracles at shrines and to live as a prostitute in the old silk house where “every swain and his father knew I was there for the taking.”

But through it all she imagines Queen Maria Luisa; because she was born at the exact time Francisca was and was precipitously married in the Luarca family’s hometown, Francisca thinks of her as a kind of twin and soul-mate, a female companion in misery. She fantasizes herself in the sorrowful palace, watching Queen Maria fight with her fierce mother-in-law, befriend the famous dwarfs of the Spanish court, avoid “the hour of wifely obligation” by playing late night games of trocero and piquet. “Was she stupid?” Francisca thinks. “Was the new queen entirely, even willfully naive? Without betraying any worry, Maria began to misbehave. She did things for which she would not be forgiven. She made the wrong enemies. Some people do.”

Kathryn Harrison’s “Thicker Than Water” (1991) and “Exposure” (1993) were harrowing contemporary novels, so it’s gratifying to find that in this book she’s handled the forbidding obligations of historical fiction so well. Harrison acknowledges guidance in her research from such institutions as the Hispanic Society of America, the National Health Museum, the Prado and the Textile Museum Library, and none of that good learning has gone to waste. She gives elegant lessons in how silk is made, how human anatomy was fleetingly taught in the age of chirurgeons, how the aristocracy so sought loftiness that they often stood on stilts, how stinging blister beetles are ground into cantharidian powder, a poison whose tincture is known as Spanish fly.

“Poison” is a fascinating, feminist princess-and-pauper story, gorgeously written and hauntingly told. It is a tale of passion, hopelessness and thwarted ambitions in a harsh and hate-filled century that was, as in all fine historical fiction, quite different than and disturbingly like our own.