Carey Mulligan stands centre stage, in a sky-blue box, and begins to tell us her story. She's warm and engaging, confident and charming, and the tale she tells is full of humour. Its first line is on the posters: "I met my husband in the queue to board an easyJet flight and I have to say I took an instant dislike to the man."
As the story of their relationship unfolds, Mulligan plays it with the skill and timing of a stand-up comic. But there's something in her eyes, some sadness behind the smiles, that make you realise this is not going to be a fairy tale of romance fulfilled. It is after all, a play by Dennis Kelly, whose preoccupations with violence and its terrible effects chart all his work – even, you could argue, in his all-conquering hit musical Matilda where the sadistic treatment of the heroine by teachers and parents is given due weight.
Here, something unthinkably terrible happens to change the tone and tenor of Mulligan's monologue. It is hard to write about because the shock is carefully and realistically contrived and withheld for a very long time. It will spoil your experience of this fine play if I reveal what Kelly deliberately holds back, letting your mind whirr round various possibilities.
What I can say is that it is profoundly upsetting and serious but conveyed with the same precision of language and delivery as the more entertaining elements. "This is the hard bit," says Mulligan, described only as the performer, as her face sets and her hands fall to her sides. All animation drains from her as she talks to a silent theatre in matter-of-fact tones.
This is an extraordinary performance. Her work on television and film makes it easy to forget what a charismatic stage actress Mulligan is, full of understanding of the way that a flex of the neck can convey the movement of a supermodel, or a quick turn of the head suggest a heart that is broken. Bare-footed and with a south London accent, she creates a fully-rounded character, a woman surprised by her own success, gratified by her own ambition, and determined not to be destroyed by tragedy.
She gives the play the emotion its gimlet-eyed descriptions notably lack. The play is deliberately cold-hearted, and I sometimes resisted its determination to be so flatly factual. But the subtlety of Mulligan's performance pulled me along. She is supported by the accuracy and power of Kelly's writing, which is in evidence even in the gritty asides – "The terrible secret of all human endeavour might just be that it's not really that hard" – as much as in the main plot.
And the picture he builds is infinitely assisted by the control of Lyndsey Turner's direction, perfect in pace and pitch, and in the way it blends the real and the remembered. Es Devlin provides the most extraordinary sets, which appear at intervals, real rooms bleached out to the same sky-blue colour as the front-stage space, with occasional objects picked out in sharp reds and oranges, like the flecks of memory.
Here, Mulligan communes with her past. The conclusion, when she faces down the sheer difficulty of her present, the courage required simply to go on, is overwhelming.