These days the Royal Court makes almost anyone feel old. Warm on the heels of 24-year-old Alexandra Wood's extraordinarily assured debut, The 11th Capital, comes this immensely promising drama of affluent yet dysfunctional family life from 20-year-old Polly Stenham.

I say promising, rather than anything more rapturous, because this is a play that from second to second displays every mark of a fine playwright without ever gaining much in the way of momentum.

Martha is an alcoholic locked in a mutually dependent Oedipal relationship with her son, school drop-out Henry. When daughter Mia uses Martha's Valium to poison a fellow boarder at her school, the authorities are alerted, and the children must decide whether to help or condemn their negligent mother.

In the round, on a clinical, white stage Stenham dissects and traces the dynamics of this messed-up family unit with maturity and subtle, characterful wit. She's helped by some seriously good performances.

Felicity Jones, a young actress acting younger, finds the fragility in the sassy Mia, her frightened eyes belying the self-assuredness of the young teen.

As Martha, Lindsay Duncan has the manipulative passive aggression of the practised addict. Her attachment to her son is insane. At one point, suspecting he has slept with a girl, she cuts up his clothes into tiny squares. They litter the stage for the duration. Their shambolic, decorative effect reflects Martha's strangely attractive distress, which on one level, at least, contrasts favourably with the stiff pragmatism of Henry and Mia's broker father.

Still, once the characters are established, the drama doesn't so much develop as play itself out. The plot unravels pleasingly and unsurprisingly as Stenham overindulges her talent for dialogue, talking through situations three or four times when once would do.

The play eventually focuses on Matt Smith's Henry, who stands at the centre of the conflict between mother, father and sister.

However, Smith's intense performance is as often inexpertly over-emphatic as it is compelling. When his time comes to make a choice, his decision feels neither in doubt nor significant, despite a lot of shouting. The drama peters out but you remember the talent of the dramatist.

• Until 19 May (020 7565 5000).