By Susan Swan

Bloomsbury. 302 pp. $23.95

In What Casanova Told Me, the Canadian novelist Susan Swan employs a venerable device: the interwoven narrative of past and present quests. This is the device perfected by A.S. Byatt in her memorable novel Possession, where two modern-day English academics try to discover the truth about a pair of 19th-century poets. A risk with this structure is that readers will find themselves more diverted by one narrative thread than the other. Byatt's book was unusual in that it balanced the tension between past and present perfectly, managing to develop two equally intriguing love affairs and two equally vivid and authentic settings.

Swan's novel doesn't achieve that fine balance. It opens in present-day Venice, where a young archivist named Luce has a dual mission: to deliver some rare, recently discovered family documents to the Sansovinian Library and then to travel on to Crete for a memorial for her mother, a feminist scholar of the Minoans. She is traveling with her mother's gay lover, a university teacher named Lee Pronski, whom she dislikes. Luce, in her twenties, is a petulant loner who hasn't shed her adolescent sense of grievance. She harbors a childish resentment over her mother's affair, which had ended their long mutual dependence. The present-day narrative charts Luce's journey through these feelings to a state of adult acceptance from which she is finally able to offer and accept love herself.

The documents she carries contain the other, historical narrative. They are the papers of Luce's distant great-aunt, Asked For Adams, Asked For being a not-unusual Puritan name among the Experiences and Thankfuls of the period. The papers include a travel diary in Asked For's own hand, a collection of letters from Jacob Casanova, whom she met while traveling in Venice with her father in 1797, and a mysterious bound volume in Arabic script. Although Luce's family has gone to the trouble of having Harvard experts authenticate the documents, no one has bothered to translate the Arabic (which actually turns out to be Turkish) before sending the papers away. This, to me, is just the first of many improbabilities that mar the narrative. Swan needs to build some suspense about the Turkish manuscript, but this is a clumsy way to do it.

Clumsy and improbable, also, is the way Swan shifts her time frame between the two interweaving narratives. We learn the story of Asked For and Casanova in snatches, as Luce finds time to read it. Surely any professional archivist worth her salt, getting her hands on material as fascinating and rare as this, would simply lock herself in a room for as long as it took to thoroughly peruse it. Luce, however, makes an achingly slow business of it, hauling copies of the papers with her through Venice, Athens, Crete and Istanbul.

Which is not to say the book lacks pleasures. Swan has tried to do a lot in What Casanova Told Me, and the tale is rich in interesting digressions into subjects as diverse as Minoan goddess worship and Western Orientalist stereotypes. Swan also has much to say about the emotional risks required to live a fulfilled life, and she can be deft in describing people. Lee Pronski falls asleep in an Athens agora, "her Birkenstocks sticking out from under the diaphanous fabric of her sun wear like the half-shod hooves of a weary draft horse." Less successful: the description she allows Asked For to give of Casanova's "manhood." (Is there, by the way, a more dispiriting term in the lexicon of historical fiction? "Ample bosom," perhaps. . . . ) It was, we're told: "as thick and long as the morels I once found under the maple trees of Quincy." Oh, please. If I ever found myself approached by a partner with a penis that looked anything like a morel, I'd be sending him to the emergency room.

In fact, it is in descriptions of Casanova that the book reaches both its greatest highs and most lamentable lows. When Asked For meets the old chevalier, most of his life's big adventures are behind him. He had been imprisoned by the Inquisition, and, before his daring escape, inscribed his famous graffito on the prison walls: "I love. Jacob Casanova, 1756."

Casanova would have been 72 in 1797, and one of Swan's intentions in the book is to show how a man of that age can still be seductive enough to draw a young woman out of her safe and stultifying world and into a life of reckless adventure. Unfortunately, she seems to lack conviction here. I found myself wondering if Swan had ever known any sexy older men. Her Casanova is too often portrayed as a wheezing geriatric wearing out-of-date clothes, and not as the character he needs to be -- vibrant, energetic and with a personal magnetism that renders any ravages of age invisible -- to make us believe he could so completely beguile and satisfy a young woman like Asked For.

Swan prefaces her novel with the disclaimer from Nathaniel Hawthorne's The House of the Seven Gables, in which he claims "a certain latitude" to depart from fidelity "to the probable and ordinary course of man's experience." Swan seems to be asking for a similar suspension of disbelief, and readers who are untroubled by improbabilities in their fiction will probably find diversion in the tales that Swan's Casanova has to tell. *

Geraldine Brooks is the author of the novels "March" and "Year of Wonders."