If you think the ruthless politics and the bloody diplomacy of Game of Thrones is pushing it in terms of dynastic power play, the toxic relationships and motives before, during and after the ill-conceived marriage of the Catholic Marguerite de Valois, daughter of Henri II and Catherine de Medici, to the Protestant Henri de Bourbon, king of Navarre, on a platform in front of the great facade of Notre Dame on 18 August 1572, are the stuff of nightmare - though also of novels, epic poetry, theatre and movies.

On 24 August, two days after the stage-managed murder of the Huguenot diplomat Gaspard de Coligny, advisor to young Henry, the streets of Paris were awash with the blood of the Huguenot wedding party, hunted down and slaughtered The killing of five to six thousand Protestant victims and others simply caught up in the violence, took a week. Though historians may hold other interested parties as partially culpable, it is generally granted - and in this book, Nancy Goldstone agrees - that Catherine de Medici, dowager queen of France after the death of her husband in 1559, was almost wholly responsible, complicit with her son, Henri III, for the horror of the St Bartholomew's Day Massacre.

As an accomplished power-broker and devout Catholic, Catherine is alleged to have arranged the wedding in order to provoke rather than heal the bitter political and religious differences of the time. Her beautiful, wilful, youngest daughter, known to everyone as Margot, was the irresistible bait that would lure Catherine's powerful and politically adroit enemy Coligny and his Protestant faction to their doom. It is doubtful she quite reckoned on what might happen afterwards. Her sole concern was the security of herself and her son Henri. She certainly gave little or no thought to the consequences for Margot, the new Queen of Navarre, who knew nothing of her part as a pawn in her mother's underhand political game and was sincerely terrified during the massacre. Though forced into a marriage against her will and religion, Margot understood very well that her own security depended on Henry of Navarre, who had survived the massacre, rather than her duplicitous mother who had used her as a dupe. She chose to protect Henry and, thus, herself.

It is worth remembering that Margot and Henri were very young. On the day of their wedding, she was 19, he was 18. She was about to lose trust in her mother, he was about to lose his mentor Coligny and most of his friends. Henry was miserable at the French court where, though he made a show of attending Mass and his marriage was legitimised by the Pope, he was disregarded when he was not taunted as a virtual prisoner. He longed to be back safely in Navarre. Without reason, he distrusted Margot who, as a young married woman, prospered as a patron of the arts. She was beautiful, she was intelligent and she learned how to be discreet in her pleasures while Henri was away on manoeuvres with the royal army. She was, in short, glamorous.

Only after some six years of marriage, during which she was either a focal point or regarded as an accessory of court intrigue, her loyalties to her brothers (successively kings of France) and her mother questioned, was Margot allowed to leave for Navarre. Henry had already managed to flee France to his small kingdom and he was in need of his wife for only one reason - to give him children. With a lavish entourage, including 20 beautiful ladies-in-waiting specifically chosen to dazzle Henri's eye, Catherine accompanied Margot to Navarre. Nevertheless, the twin irritants of infidelity and religion were settled more or less amicably between Margot and Henri who lived more or less happily together until ... the rest is a story of love's sweet but indiscreet rapture when Margot's affairs lost her the support of her brother Henri, the French king, Henri her husband, and Catherine her mother. Margot was never able to retrieve her situation. In 1599, six years after Henry of Navarre became King of France, Margot's marriage was declared null and void by the Pope. She was given the honorary title of queen as well as duchess of Valois, and lived quietly and well.

"Against all the odds," writes Goldsmith, "she had survived the murderous brutality of her family and her times. She was free of a marriage she had never wanted, that had taken place under false pretences ... In this she embodied France itself." But for the fact that she never gave her husband a child, Nancy Goldstone claims that Margot could now have been regarded by history as one of the great French rulers. Just as the self-serving, Machiavellian reputation of Catherine de Medici is occasionally rationalised as pragmatic and her power-broking in the game of thrones as politically astute, so Goldstone attempts to upgrade Margot's reputation as a sparkling libertine who put love and sensuality above her duty as queen. This account of the turbulent years between Margot's marriage in 1572 and her death in 1615 diligently presents Margot as resilient in her own (and her husband's) interests, defiant of her controlling mother and, in modern terms, a proto-feminist, "a strong, spirited, resolute, individual unafraid to confront sexual mores. ... Alone among her family, Margot refused to use sex as a weapon and searched only for love."

Whether Goldstone succeeds in her special pleading, it is a case made with great feeling throughout a colourfully dramatic narrative that presents not only the internecine family of Catherine de Medici as vicious and opportunistic, controlled by a relentlessly calculating queen, but Margot as the one who got away and, though perhaps not in her own terms, lived relatively long and prospered pretty well.