Mud, mud, glorious mud. Vicky Featherstone's production of Simon Longman's play Gundog is practically drowning in it. A mountain of the brown stuff towers in the corners of the stage in the Jerwood Theatre Upstairs at the Royal Court. It smells musty and a bit, well, earthy, though it's anything but wholesome. Chloe Lamford's designs posit the mud as a malignant, lingering presence, something which hangs over the lives of the family at the play's centre, threatening to engulf them. They can't escape the dirt.

It's often the way with farmers. Mud and blood get under the nails and can't be cleaned out. Here it's shepherds Becky and Anna, one always carrying a shotgun, in their filthy baggy jumpers and wellies. The elder Anna is trying to keep the farm, and their lives, afloat by tending to what's left of the flock after their parents die and their elder brother leaves them. The years roll on, lambing season after lambing season, dead ewe after dead ewe, their lives in thrall to the only job they know how to do.

The play works backwards and then forwards, opening with a central moment in the narrative: a meeting with foreign rough sleeper Guy who is discovered by the women in the middle of nowhere and is taken in by them. He begins to work with them without payment, but in return for something to do and somewhere to live. The play jumps forward: we see how he settles, how he fits in, and then back: when their long lost brother Ben returns to the farm we are shown why and how the two sisters were left alone.

It's a brittle story, one that's cold and relentless, like the lives of these land toilers. But it's also engrossing and Longman deftly works poetry into the drudgery. Several monologues are revelations: the old grandfather, sinking into dementia, has one final lucid moment and tells the siblings he loves them all; when Becky chats with Guy about how he must reconnect with his parents. They are moments of raw truth delivered by characters who have nothing but each other and the fields they work. Gundog is essentially the story of a family finding its way, but it tells that story in a tricksy, non-linear direction, which makes it all the more real.

Featherstone's production keeps things bleak, but not unbearably so. She plays up the sense of passing time, leaving only a light change to denote an entire year slipping by. Peter Rice's heavy sound design features bursts of deafening white noise which suggest the torment within, and contrasts with the uncomfortable silence of the wilderness in which they work. The cast are excellent, with Ria Zmitrowicz superb as Becky, imbued with the fearlessness her grandfather tells them all, the year before he dies, to make sure they have. Alan Williams as Mick the grandfather is funny and tragic, delivering his moments of clarity with poise and grace.

It's not a play that offers a laugh a minute, but it is a beautiful and truthful thing, a painful portrait of a family struggling to deal with the ghosts of their past and the promise of the future.

Gundog runs at the Royal Court until 10 March.