Harper Regan, National Theatre: Cottesloe, London

In his last play, Motortown, Simon Stephens sent a squaddie, newly returned from Basra, on a brutal, alienated road trip through an England that had become a foreign country to him. There's a similarly picaresque and circular structure to his new play, Harper Regan, but this time it's a female protagonist who embarks on the journey.

The eponymous Harper is a 41-year-old working mum who lives in Uxbridge. She is desperate to visit her dying father in Stockport but is denied leave of absence. So, without informing her husband or daughter, she takes off regardless but fails to arrive in time to tell her father that she loved him. As the family breadwinner, she has risked their livelihood on this abortive mission, but it eventually becomes clear that there is a darker emotional background to her flight from home.

Stephens has written a terrific central role and Lesley Sharp, making a welcome return to the stage, gives a superb performance that runs the full emotional gamut. Long before the revelatory outburst, she subtly lets you sense the unspoken tension and hurt in her marriage to Nick Sidi's uneasy Seth.

In her impulsive Northern truancy from these pressures, she has encounters with lonely males. Pestered in a pub at 11am by a coke-snorting, anti-Semitic journo, she dangles him with mockingly contemptuous fake interest. Edging towards sex with a married man in a swanky hotel, she communicates a shy, aching need for intimacy. At her estranged suburban mother (a splendid Susan Brown) she directs a coldly blazing anger. She can't forgive the woman for trying to undermine her trust in her husband, but in the barely controlled fury of their session together, she learns things that jolt her attitude to him.

In Marianne Elliott's compelling production, the various figures assemble round Harper with a dream-like inevitability. Stephens's writing beautifully depicts the shift from a love based on dogged faith to one that can survive bleak realism. Harper can't get inside her husband's head and police his thoughts. Admitting this and other limitations to her defensive daughter (a first-rate Jessica Raine) may help break a bad family pattern of invidious allegiances. And her brave, steady honesty becomes the basis for a return to tenderness in the touching ceremony of breakfast in the sun that offers a climactic hint of redemption.

To 9 August (020-7452 3000)