In her powerful and original book, Captives (2002), Linda Colley recovered the hidden histories of Britons seized and taken into servitude in the 18th century, describing them as "the underbelly" of the emerging British Empire.

Drawing on dozens of neglected first- person accounts of colonial travellers in trouble, she revealed the harsh, dangerous world of the small tradesmen, ship's captains and, occasionally, merchant's wives carrying out their duties abroad on behalf of their homeland, and the sombre reality of life at the bottom of the pile for those taken into captivity in territories as far apart as Algiers and Afghanistan. Their largely unfortunate adventures overseas, she suggested, provide a telling counterpoint for the traditional, triumphalist narratives of conquest and territorial expansion we associate with the Age of Empire: "English, Welsh, Scottish and Irish men and women, seized in successive captivity crises overseas, mark out the changing boundaries over time of Britain's imperial aggression, and the frontiers of its inhabitants' fears, insecurities and deficiencies."

In The Ordeal of Elizabeth Marsh: A Woman in World History , Colley singles out one of those gripping stories of abduction and imprisonment she unearthed in Captives - the only published account by a woman - and fills out the protagonist's history, as far as she is able, into a complete life.

Elizabeth Marsh was the child of a ship's carpenter and the white Creole planter's daughter he had wooed and won while on a naval tour of duty in Jamaica. Like Mr Rochester's wife Bertha Mason in Charlotte Bront?'s Jane Eyre , Marsh's mother brought with her to England from the Caribbean the attraction of plantation wealth and apparent breeding, but also the "taint" (to which Colley returns at several points in the book) of possible racial mixture and emotional instability. Marsh's parentage indelibly marked her own destiny - an insecure life of wandering, uncertainty, social dislocation and a continual, unsuccessful quest for financial stability.

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As a young, single woman, while travelling by ship between Menorca and England in 1756, Marsh was captured by Arab corsairs and taken as a hostage to the Sultan of Morocco, Sidi Muhammad. Fortunately for her and her fellow male captives, their detention was a diplomatic matter. The Sultan was demanding an apology for the insulting behaviour at his court of a reckless English diplomat dispatched there by the British Government to negotiate a treaty. Once the diplomatic row had been settled, the Sultan released the hostages he had grabbed as bargaining chips.

Marsh, however, knew nothing of the political finer points of her situation. As far as she was concerned, her predicament was dire and she was in acute danger of being detained for life in the Sultan's seraglio - a fate that seemed the more certain after she had had two private audiences with the Sultan himself.

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One of her fellow captives, James Crisp, volunteered to masquerade as Marsh's husband, thereby saving her from sexual servitude. Once safely back in London, she reluctantly married Crisp to save her reputation. For a short time, his somewhat shady overseas business ventures flourished, and she enjoyed a high society life in London. Marital affluence, however, did not last. Thirteen years later, when family fortunes were at a low point, Marsh wrote up her Moroccan adventure and published it in two slim volumes as The Female Captive , achieving some small success and renown from this unashamedly romanticised account of her fashionably exotic adventure.

This was not, however, the end of Marsh's encounters with the global repercussions of English imperial ambitions. After her husband had made and lost a small fortune in speculation in London, the couple embarked on a new life in India. When Crisp lost his job with the East India Company because of his neglect of company business and his private dealings on the side, his wife set off, scandalously, on an 18-month tour of the east coast of India in the company of a younger, unmarried "cousin". Returning to find that Crisp had died, she chose to remain in India and on her own death was buried in Calcutta.

Marsh was a fundamentally ordinary woman, obliged by circumstances beyond her control to survive by her wits in a surprising variety of locations outside her native Britain. What is perhaps most interesting about her as a person is the way, as a relative nonentity from a socially mediocre background, she could nevertheless deftly talk her way into elite treatment whenever she found herself hard-pressed abroad.

No matter how her circumstances declined, Marsh always somehow contrived to get herself treated as a "lady", thereby managing to escape some of the direst possible outcomes that might otherwise have befallen her. She was, as Colley more kindly describes her, "socially obscure, sometimes impoverished, and elusively mobile".

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Marsh spent a lot of her life pretending. Her first-person accounts shape her experiences to the contours of contemporary sentimental fiction and are designed to show herself in the best possible light. Those around her, on the whole, failed to pay much attention to her at all, it seems - apart from a few disapproving remarks by a considerably more successful uncle penning his own family history.

In the end, the interrupted traces of her picaresque journey in pursuit of a tolerable life are curiously uninformative about Marsh herself - not so much as a portrait of her survives, and both her published account of her captivity in Morocco and her Indian diary are tantalisingly short of emotional detail and colour.

But by following Marsh's story, Colley convincingly argues, we learn a great deal about the British imperial milieu in which she moved. Marsh's story was "poised on a cusp between phases in world history". It allows us to begin to understand the relationship between the "impersonal and remote transformations" of world history and "the most intimate features of the human self", Colley says.

As Elizabeth Marsh herself moves incomprehendingly through her life, the seismic shifts in power structures - social, economic and political - of British imperial history are brought into sharp focus via our picaresque heroine, and converted to a more human and graspable scale. Here, indeed, is a woman in world history.

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Lisa Jardine is centenary professor of Renaissance studies and director of the AHRC Centre for Editing Lives and Letters at Queen Mary, University of London.

Lisa Jardine finds a personal history that illuminates imperial Britain

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