The Children's Hour
Comedy Theatre, London SW1

I can see why audiences could have trouble with this play, which was written by Lillian Hellman in 1934 when she was just 28. Its apparent subject, lesbianism, is no longer regarded as what the mayor of Boston called "moral perversion" when he banned the play in his city in 1935. The motivations of Mary, the girl who accuses her teachers Karen and Martha of homosexuality, are obscure and unexplored. The play's focus shifts abruptly between acts, from Mary and her cackling school pals to Karen and Martha. The schoolgirl scenes struggle to convince when the actresses appear to be well into their twenties. And the ending, oh, the ending! I blame Chekhov.

But I loved The Children's Hour almost as much for its imperfections as for its stretches of bracing, hate-filled brilliance. Ian Rickson's powerful West End revival, sponsored by American Airlines (will it fly Boeings full of New Yorkers to London for it?), is a vehicle for its stars, Keira Knightley and Elisabeth Moss. The pirate of the Caribbean plays Karen, fiancée of a stout-hearted doctor and co-founder of a modest new school. She is the calmer of the cynical duo. Physically, Knightley is willowy to the point of being blown away but, psychologically, she makes Karen an oak. Until the end, she demands that the truth be a sacred value. Only Knightley's American accent wavers, but it doesn't matter. She is strong and strongly cast.

It is Moss as Martha, however, who is the elemental force: she is full of righteous anger against the injustice of the allegation and then, in a game-changing aria towards the end, swept up in a gale of emotional incontinence. Mary, Martha tells unsuspecting Karen, chose the lie that had the ounce of truth. Make that a tonne. In Mad Men, a series whose acting standards are on a level all their own, Moss, as the ambitious copywriter Peggy, has helped to make some of the best scenes. But as Peggy, Moss is quiet and suppressed; she is a corked bottle of resentment. When I first heard that she had been cast, I feared that her voice might not carry. How wrong I was. Moss fills the auditorium. As she took her bow, beaming, I felt myself falling in love, just for all the pleasure her acting gives.

This pair are hard competition but Bryony Hannah does well as young Mary, the snake in their rough Eden. She is too old for the part but she is lithe, flopping over desks, writhing on sofas, collapsing and reassembling herself like a child. Her motives are as obscure as those of Iago or the brat in We Need to Talk About Kevin but that doesn't bother me too much. Whether through nature or nurture - her father killed himself and she was brought up by his doting mother - she is a pathological liar.

Ellen Burstyn, who plays the guardian who believes Mary and gets Martha's and Karen's school closed, is not the strongest point of the production, mainly because she seems too wise to believe such nonsense. But in Burstyn's rendition, she may be the most purely motivated character in the piece. Carol Kane (Andy Kaufman's girlfriend Simka in the sitcom Taxi) is an astonishingly fey drama teacher whose penchant for sins of omission and outright fibs completes the company of she-devils.

Tobias Menzies, as Karen's faithful fiancé who finally cannot resist asking her if the lie is true, has perhaps the most difficult task of all: convincing us that a man of humanity and loyalty can be corrupted by a schoolgirl lie. He takes us a long way towards that point.

The play was considered shocking when it toured America in the 1930s. It may not be so shocking now, but it still causes a frisson because of its insistence that childhood can nurture wickedness. Yet its true subject is the potency of the well-targeted lie. Of its author, the writer and activist Mary McCarthy once said on a chat show: "Every word she writes is a lie, including 'and' and 'the'." Knowing that adds something, I should say.

“The Children's Hour" is booking until 7 May
Andrew Billen is a staff writer for the Times