By Emma Donoghue. Harcourt. 650 pp. $26

Who can resist the seductions of a lush period drama? The parade of fabulous clothes, the genteel manners, the veiled eroticism that always seems to burst forth in a breathless flurry of petticoats and wigs in some broom closet. Repressive social mores and constricting dress make for a fertile dramatic backdrop -- one that's ripe for a scandal. Irish writer Emma Donoghue plumbs this territory in Life Mask, her mesmerizing new novel, which at 650 pages is like one of those great 19th-century tomes that you're sad to see come to an end. Donoghue's approach with her two most recent books -- the bestselling novel Slammerkin and the provocatively titled story collection The Woman Who Gave Birth to Rabbits -- has been to spin an intricate fictional web around bits of historical marginalia. Among the documents that inspired Slammerkin, for instance, was a broadsheet from 1764 that claimed that a servant girl named Mary Saunders, hanged for the brutal murder of her mistress, was driven to it by a lust for fine clothes. Donoghue recognized that tantalizing trifle of information as literary gold and developed it into a Dickensian tale of a penniless child whose intense longing for a red ribbon led to a life of dissolution and crime.

With Life Mask -- set in the waning decades of the 18th century, the French Revolution raging in the not-so-distant background -- Donoghue shows that she is equally at home in the drawing rooms of the Beau Monde and the squalid cellars of poor working-class London. Training her eye on the lavish lifestyle of the "glittering throng," she dissects the very world that Mary Saunders yearned for but could not reach. At one point in Slammerkin Mary contemplates making a clean start and maybe becoming an actress -- "or a rich man's wife. Something that lets me wear silk all day. Something to lift me above the mob." Donoghue's new heroine, the lovely but low-born Eliza Farren, who rises to prominence as the Queen of Comedy at Drury Lane Theater and marries an earl, is the very embodiment of poor Mary's dreams.

The cast of characters (all real people) listed at the back of the book spreads over six pages and includes such notables as the fiery Whig leader Charles James Fox, "Prinny" (the profligate Prince of Wales), and the legendary society beauty and Whig campaigner Georgiana, Duchess of Devonshire (who was the model for Lady Teazle in Richard Sheridan's "The School for Scandal"). Donoghue's story focuses on a less historically prominent threesome that moved in the same elite circles: the fantastically rich and ugly Lord Derby (for whom the horse race is named); his longtime inamorata Eliza Farren; and the widow and accomplished sculptor Anne Damer. Anne and Derby are childhood friends, and it is with Anne's help that Derby hopes to introduce Eliza, an actress with no pedigree, into the fold of fine society.

Derby's romantic predicament is unlike that of other married men in his set, who pursue dalliances as unabashedly as they bet on horses. But Eliza's fear of being marked as a kept woman has led her to impose a set of draconian limits on the relationship -- no gifts, no excessive flattery or familiarity in public, and private meetings permitted only in the company of her mother -- all of which Derby has stoically accepted. His long-suffering devotion is a source of endless amusement for the scandal sheets. As Derby had hoped, Anne Damer takes an instant liking to Eliza, and the two become fast friends, paving the way for Eliza's acceptance by the World -- the Beau Monde. But the women's mutual regard can't withstand the rumors of Sapphism, that "unnatural vice," which have been dogging Anne ever since her husband's suicide. The latest circulating epigram is too much for Eliza to ignore: "Her little Stock of private Fame/ Will fall a Wreck to public Clamour,/ If Farren herds with her whose Name/ Approaches very near to Damn her. "

Political fireworks backlight these private dramas. The liberal Whigs, led by Fox, are trying to wrest power from William Pitt's Tory government, which has the support of the deranged George III, and to usher in reforms. Derby and his friends are ardent Foxites (though Derby draws the line at renouncing his title and stripping his carriage of the family crest, and Anne and Eliza can't quite bring themselves to give up sugar for the anti-slavery campaign: "What would one serve for desserts, apart from oranges and walnuts?"). But the escalating violence of the French Revolution breeds fear of domestic revolt among the upper classes and in government and drives a wedge into the Whig party. The gist of the clashing political viewpoints may seem familiar: "This is our civilisation's stand against an enemy of a kind we've never encountered before. . . . They aim to spread their infection of anarchy till all Europe is one howling furnace." The rejoinder: "I'm more afraid that Pitt and the King, on the pretext of national security, will destroy everything I love about England."

Donoghue, who is also a playwright and historian, has alighted on another terrific story, and she pulls off a dazzling feat of choreography in setting it all in motion. She takes obvious delight in the sumptuous details of dress and comportment, the subtle inflections in conversation and the slow blooming of erotic tension. As Georgina, Duchess of Devonshire would say, "It was all simply ravish."

Julia Livshin is a staff editor at the Atlantic.