The Children's Hour, Comedy Theatre, London

Having routed the nay-sayers with her elegantly snaky portrayal of a backbiting bitch in The Misanthrope, Keira Knightley is back on stage. This time she is teamed with Elisabeth (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 malicious pupil starts a false rumour that they are lesbian lovers.

In a play of very mixed merit artistically, Moss makes a powerful impression as Martha, starting off all wittily abrasive arms-akimbo defiance then succumbing to humbled selflessness, as she is forced to confront the lesbian feelings she had hitherto repressed.

Knightley's Karen begins as a focused woman, elegant in period bob and pencil skirt. Her performance is at its best in the scene with her fiancé (Tobias Menzies) when she realises that she will never be sure that he has managed to overcome all doubt. The actress's manner here wavers most convincingly between angry touchiness and tearful tenderness as she releases them both to their separate freedoms. By contrast, she is wooden during the post-suicide disarray and stagy during the ferocious rejection of the grande dame grandmother (well played by Ellen Burstyn) who spread the fatal rumour.

The one astonishing performance, though, is that given by Bryony Hannah as the pupil, Mary. Imagine that brat who plays Katharine Hepburn's sister in The Philadelphia Story possessed by an unappeasable demonic hunger and you'll get some idea of the black comic force of this Mary's self-dramatisations among the adults and bullying destructiveness among her peers. Knightley was nominated for an Olivier for her performance in The Misanthrope. If anyone wins an Olivier for The Children's Hour it will be Ms Hannah.

To 30 April (0844 871 7622).