As with the Holocaust, unless your family history is enmeshed in it – and possibly not even then; I am fortunate enough not to know in either case – it is impossible to comprehend the scale of suffering involved in the transatlantic slave trade. And easy to look away if someone tries to teach you.

Andrea Levy’s genius in her 2010 Man Booker shortlisted novel The Long Song was to distil its essence – with a lightness of touch that enabled the reader to face the horror without effacing it – into an intimate human drama. The Long Song presents the life story of July, who is writing the book as her memoir. Conceived by the rape of her mother, Kitty, by the overseer on the plantation on which she worked as a field slave, July is taken from her at the age of eight by the plantation owner’s monstrous sister Caroline, to be trained as her lady’s maid – once she has been renamed something more suitable: Marguerite. She grows to adulthood during the last – official – days of slavery in Jamaica.

The opening episode of last night’s three-part adaptation, to be screened over consecutive nights, manages the same feat, thanks to a finely whetted script from Sarah Williams (who also adapted Levy’s Small Island for television in 2009) and some outstanding work from a first-class cast. Central to this is rising star Tamara Lawrance, who captures all of July’s ebullience and intelligence, fiercely restrained in the capricious, violent mistress’s presence but forever straining at its bounds. Over the years she learns to handle Caroline (a pitch-perfect performance from Hayley Atwell, who takes her right up to the line of real monstrousness without crossing into caricature), make a good life within its awful and motherless constraints – and then, gleefully at first, embraces the upending of that life when the Christmas Rebellion begins.

The house slaves have been hearing rumours that the king of England is minded to bring an end to slavery. They hear them as they prepare for a madly elaborate Christmas dinner that the plantation can ill afford and that Caroline, sick of her penury, is insisting on. “A hundred beeswax candles for the walls!” she cries. “I’ve seen it done in London.” July’s fellow house slave Godfrey, superbly played by Lenny Henry as a man with a lifetime of suppressed rage etched in every lineament, moving slowly through his duties as if the hatred is ballast in his very bones, tells her what this will cost. She accuses him of cheating her – it cannot be that expensive. “It’s not that the candles are expensive, Missis. It is that you cannot afford them.” The stifled laughter of the other staff turns to fear in an instant as she strikes him and threatens a whipping. Even tiny moments of freedom have their reckoning.

Larger ones come swiftly. The dinner is interrupted by a call for the men to saddle up and help put down the fighting that has broken out in the west, and for Caroline to be taken down to the ship that is taking the women to safety. Godfrey demands and exacts cash payment for doing so. Free of the master and mistress for the first time in her life, July eats, drinks and makes extremely merry (also for the first time with her inamorata, Nimrod, who bought his freedom and promises her a life of liberty in a house he owns once the island has ended its convulsions).

But they are discovered in the master’s bedroom after he kills himself there and Nimrod is framed by Caroline as a murderer. Impossibly harrowing scenes follow that illuminate the inhumanity of man at every turn: of their escape to the field slave quarters, where July is briefly – heartstoppingly – reunited with the mother she presumed dead or sent away and then – heartbreakingly – sundered from her; Nimrod’s death; and finally Kitty’s execution for killing the guard who was about to murder July. “They hanged so many the pile [of bodies] began to interfere with the drop,” records July in her memoirs. July is sent back to the fields until Caroline needs her again. She returns to the house, but unrest elsewhere continues until the king announces that the colony must start preparing for “the beginning of the end” of slavery. A new overseer arrives, joyfully announcing that: “In a few days that dreadful evil will be over,” and greeting both lady and maid of the house with an equally cheery smile.

“If only,” comes the voice of July, “my story were so simple.”

A beautiful, moving, horrifying adaptation of her unsimple tale, that honours the source and its subject.