Theatre Review: The Herd - Rory Kinnear's debut play is well acted but a little forced

The Olivier Award-winning actor Rory Kinnear has pulled off the neat trick of being in two places at once in London's early autumn theatre season.

At the National, he is a superb Iago to Adrian Lester's Othello. Meanwhile, he is making his debut as a dramatist at the Bush Theatre with The Herd, a witty, agonising and eye-opening play which draws on his experiences as the sibling of a severely disabled older sister.

The format is a family gathering – a twenty-first birthday party – with the twist that Andy, brain-damaged and physically incapacitated from birth, has a mental age of ten months. As they await his arrival from the care home, the play reveals, with clarity and compassion, the profound emotional impact that the constant fight to look after him has taken on his nearest and dearest.

Amanda Root is wonderfully embattled as his mother, Carol, whose life has been monopolised by the struggle since she was abandoned by her husband when the boy was five. She's snarling and scornful when he (Ian, played by Adrian Rawlins) gate-crashes the occasion, seeking a fresh start with his son. "It's the guilt you can't live with," she says. "Not Andy you can't live without."

But it's telling that, with Andy's best interests at heart, she's prepared to let Ian stick around in case there's a flicker of pleased recognition from the birthday boy. By contrast, Louise Brealey, excellent as their 33-year-old daughter, Claire, reacts to Ian's presence with a gut-wrenching mix of fury and need, still wounded by a sense of rejection and the unfairness of having being required to take on the role of second parent to Andy.

The piece sometimes feels a bit awkward, as though it has set out on a conscious mission to instruct. Nevertheless, its observations on how extreme disability can call forth all-consuming devotion, and on the intractability of the blame game that then ensues with those who feel they have been sacrificed, are keenly felt.

Director Howard Davies's beautifully acted production does rich justice to the weave of painful insights and playful humour. Kenneth Cranham exudes gruff warmth as the grandfather who himself needs help with moving around, and Anna Calder-Marshall brings a deliciously sly comic timing to the part of Patricia, the imperious grandmother who reacts to the news that Claire's boyfriend (Adrian Bower) is a performance poet with a lovely little flinch of sceptical forbearance.

To October 26: 020 8743 5050