Don't let Tony Kushner do your washing. The offbeat American playwright tends to shrink things down to size. Angels in America might seem expansive, but such is its sweep, its eight hours are mighty economical. Even so, Caroline, or Change has it trumped. A remarkable, unblinking chamber musical, seen at the National a decade ago, it manages to condense the core of the civil rights movement down to an argument over laundry.

Small change, that's what it boils down to. Sharon D Clarke plays Caroline, a black maid and mother of four earning $30 a week working for a white Jewish family in Louisana, 1963. Nine year-old Noah watches her at work in the basement, sorting the linen and keeping stray coins from the wash; a way for him to avoid his new stepmother Rose (Lauren Ward).

To teach him a lesson about the value of money – and, partly, to alleviate her New Yorker's guilt at having a housemaid at all - Rose orders Caroline to keep the coins she pulls out of Noah's pockets. It's a tiny domestic wrangle that encapsulates the entire racial hierarchy of America: one child's forgotten dime is a grown woman's relief fund – a way to treat her kids or cover essential costs. The drama comes from her conscience – the onus being on Caroline to take a raise, not just to accept one gratefully, and doing so means elevating her needs over Noah's. As the stakes increase, small change becomes big bills, her laundry room fills up with voices: her kids float by, her radio sings songs of a changing world and the electric dryer (Ako Mitchell) becomes the devil on her shoulder.

As one song has it: "Change comes fast, change comes slow." Kushner acknowledges that change - real change - is incremental and complex, and moreover, it's motored by money. Against a backdrop of sweeping social reform - Kennedy's assassination comes as the civil rights movement gathers pace - Caroline embodies a submission that slows progress to a standstill. Her teenage daughter agitates for a speedier shift and Abiona Omonua plays her with an uncontrolled energy.

Clarke, by contrast, is a still centre, forever folding linen and staring out into space. She manages to be a million miles away, without ever seeming dreamy. Her mode is more like dissociation; as unthinking and automated as the mod-cons around her. She extends Caroline's unsmiling nature into a blank inexpressiveness, as if she's stopped feeling or hoping. Yet her stillness is as resolute as it is resigned - a determination to keep on keeping on. She allows herself one cigarette a day, a mark of her restraint and her self-denial.

Jeanine Tesori's score matches that stoicism, refusing the soaring highs of more sentimental musicals. Instead, it shifts slowly: Caroline's sound, rooted in work songs and plaintive prayer music, rubs up against the peppier, poppier fare of her radio (a Supremes-style trio in antenna alice bands) and her bubbly washing machine (Me''ha Bryan) that stands for the speedy change sweeping through America.

That's typical of Kushner's surrealist streak, and the musical form serves it well, shooting expressionism through stark social realities. Mike Longhurst and designer Fly Davis unleash its liveliness with a giddy staging that's kept grounded by Clarke and her deep, earthy voice. The scenario might be small. The show is anything but.

Caroline, or Change runs at the Chichester Festival Theatre until 3 June.