THE BRITISH VERSION of history glosses over the time when this country was the world's biggest drug pusher. Afghanistan now produces the poppies to supply Europe's heroin. But two centuries ago it was British fortune seekers in India who turned the banks of the Ganges into a sea of poppies and tried to force refined opium on the reluctant Chinese. They almost succeeded.

Despite the emperor's decrees banning the drug that dulled his subjects and addled his empire, British traders kept shipping out jars of opium to Canton, counting on the growing number of addicts to defy his orders. In the end, they used force - denouncing Chinese restrictions on free trade, and persuading London, shamefully, to wage the notorious opium wars.

Against this background, Sea of Poppies paints a poignant picture of the human devastation of this trade. The fertile farms of the Ganges plain are blooming only with poppies - beautiful, deadly, denying the peasants the crops to sustain them and indebting them to moneylenders and landowners, themselves indebted to the buccaneers of the East India Company. Skilfully and seemingly randomly, Ghosh assembles those who will set sail in his narrative of the Ibis, an old slaving ship that is taking indentured labourers to Mauritius.

There is Deeti, married off to a hopeless opium addict, drugged and violated on her wedding night by his brother, bullied into the ritual practice of suttee, and rescued from the flames of self-immolation by the loyal, massive Kalua, whose cross-caste elopement starts a manhunt the length of the Ganges. There is Paulette, the daughter of a French botanist, brought up with her Indian nurse's son but forced back into European pretensions of clothes, class and snobbery in the household of Benjamin Burnham, the rich, odious and flawed Calcutta merchant and archetype of British rule.

There is also Zachary, a mulatto freedman from Baltimore who guards his emotions with the secret of his birth; Baboo Nob Kissim, the superstitious and bowel-obsessed East India Company bureaucrat; Serang Ali, a wily leader of the intinerant deckhands known as lascars; and Neel, a dreamy, cultured Bengali raja whose honour, extravagance and financial naivety lead him to bankruptcy, trial, shame and sentence of deportation, as the British who dined at his table seek to grab his lands.

We follow them, through clashes of caste and custom, ruled and rulers, generous sentiment and avaricious deceit, to the fateful ship. India in the 1830s is wonderfully evoked - the smells, rituals and squalor. The language, above all, brings home the exotic: thug, pukka, sahib, serang, mali, lathi, dekko and punkah-wallah still retain, to English ears, echoes of the Raj. But the clothes - zerbaft brocade, shanbaff dhoti, alliballie kurta, jooties and nayansukh - or the ranks and offices - dasturi, sirdar, maharir, serishtas and burkundaz - are frankly incomprehensible. And that is Ghosh's trick: we clutch at what we can, but swaths of narrative wash over us, just as they did over those caught up in a colonial history they could neither control nor understand.

The pace quickens and darkens as Ghosh musters them in Calcutta and brings them aboard. Coarseness and violence, cruelty and fatalism are relieved with flashes of emotion and kindness. This is no anti-colonial rant or didactic tableau but the story of men and women of all races and castes, cooped up on a voyage across the “Black Water” that strips them of dignity and ends in storm, neither in despair nor resolution. It is profoundly moving.