James Fleet in the role exudes the dank unease of middle age, veering between officiousness at work and desperation in the evening: "Give me a joint, I've had a difficult last ten years." A pitch-perfect performance.

Richard Bean's script is also choc-full of classy rants to enjoy. In her button-up cardies and navy trousers, Stevenson carries herself with the coiled annoyance of an academic who's had just about enough of research targets and funding hoops.

But she's bossy and needs a foil, which she gets in the combat-trousered  Ben, her new first-year student. His rendition of a dopey (in every sense) 19-year-old veering between sharp curiosity and hipster argot will be instantly recognisable to anyone who has spawned a teenager since the millennium.

Bean has had a chequered time lately with a much-criticised treatment of English racism in England People Very Nice, but his awkward-squad credentials shine here. The Heretic is a play that understands how convictions are wired into fears, hopes and quirks beyond rationality. I dig, as Ben would say.

After the greenery, the glamour. Few men I know fail to worship at the shrine of Keira Knightley, the meltingly gorgeous star of Atonement and just about everything else on the big screen in the past five years. Few women don't venerate Elisabeth Moss's anxious, ambitious Peggy in TV's Mad Men. So The Children's Hour, Lillian Hellman's Thirties play of malign intrigue at a New England girls' school brings star quality to the Comedy Theatre.

Knightley, in her second major theatre role, is Karen, a prickly teacher accused by a vengeful pupil of a lesbian affair with a colleague. Shocking stuff at the time, when The Children's Hour was deemed "unfit for public consumption", rather harder to get worked up about now.

The action only comes to life when Mary (Bryony Hannah), the child-nemesis, is on stage. She's the real star here, an unhappy, sleeve-plucking sociopath who bullies her classmate into backing her story of Sapphic goings-on. As Martha, Moss deftly conveys her hidden attraction to her friend.

As tragedy unfolds, Karen laments the perfidy of the "lie with the ounce of truth in it". We never really know if she reciprocates the desire, though she seems in something of a hurry to give up her fiancé (Tobias Menzies), who is only trying to help.

One of the flaws in Hellman's creaky play is Karen's bloodless part. Knightley turns in a competent performance, but not one that arouses a swell of pity or anger at her plight. It all falls apart in the final scene when she has to do the heavy lifting — and can't sustain a credible emotional register.

Ellen Burstyn tries her best as Mary's credulous grandmother, a New England grande dame out of her depth, but the play is dead long before the final gunshot. At the curtain call, I could swear I glimpsed relief on Keira's lovely features that it was over for the night.