First Night: The Children's Hour, Comedy Theatre, London

There was a time when, to her detractors, the phrase "a second West End stage appearance by Keira Knightley" would have seemed as intrinsically absurd as, say, "a second successful performance of ritual hara-kiri". But then she routed the nay-sayers with her elegantly snaky portrayal of a backbiting bitch in The Misanthrope.

So now she is back, teamed with Elizabeth (Mad Men) Moss – in Ian Rickson's typically imaginative production of The Children's Hour, the 1934 play by Lillian Hellman in which the lives of two New England teachers are ruined when a pupil starts a false rumour that they are lesbian lovers. It's a drama renowned for a New York revival in 1952 when it stood as a telling protest against McCarthy's witch hunt. But it is a play of very mixed merit artistically.

Moss makes a powerful impression as Martha, starting off all wittily abrasive, arms-akimbo defiance and then succumbing to a kind of glowingly humbled selflessness, as she is forced by the lie and its consequences to confront the genuine lesbian feelings she had hitherto repressed. But not even a director as fine as Rickson can stop her suicide from looking like a piece of creakily motivated melodrama.

Knightley's Karen begins as a focused, professional woman, looking elegant in period bob and pencil skirt. Hellman wants us to see how the pupil's falsehood planted the seeds of mistrust not just in the parents but within its victims. Knightley's performance is at its best in the difficult scene with her loyal fiancé (Tobias Menzies) when she realises that she will never be sure that he has managed to overcome all doubt and that she is not prepared to marry him on those terms. The actress's manner here wavers most convincingly between angry touchiness and tearful tenderness as, in passages of poor man's Ibsen, she released them both to their separate freedoms.

By contrast, she is wooden during the post-suicide disarray and stagey during the climactic rejection of the grandmother (well-played by Ellen Burstyn) who spread the rumour.

The one truly astonishing performance, though, is that given by Bryony Hannah as the malicious pupil, Mary.

Imagine that intimidatingly precocious brat who plays Katherine Hepburn's kid sister in The Philadelphia Story possessed by an unappeasable demonic hunger and you'll get some idea of the unnerving, blackly comic force of this Mary's headlong self-dramatisations amongst the adults and bullying destructiveness among her peers. Rickson invents a wordless prologue in which she is seen showing a dirty book to a tumble of schoolgirls. There's a hothouse atmosphere of pre-pubescent sexuality which the production suggests is on a continuum with the hysteria of the Salem girls in The Crucible, the Arthur Miller play of which The Children's Hour is the precursor.

Keira Knightley was nominated for an Olivier for The Miser. If anyone wins an Olivier for The Children's Hour it will be Ms Hannah.