The Children's Hour, Comedy Theatre, London<br/>Plenty, Crucible Studio, Sheffield<br/>The Heretic, Royal Court Downstairs, London

Stir a spot of troubled water and you might start a whirlpool.

That's metaphorically what happens in The Children's Hour, Lillian Hellman's 1930s drama about the power of lies, suppressed desires and dangerous pubescent girls. Her script was censored by Britain's primly paranoid Lord Chamberlain at the time, but it evidently influenced Arthur Miller's 1950s masterpiece, The Crucible. And now it's enjoying a star-studded West End production, featuring Keira Knightley.

In a rural backwater, in the reactionary heart of the United States, Mary Tilford is a problem child who regularly wriggles out of punishments and chillingly contrives to rule the roost. When a fainting fit is dismissed as play-acting, she points an accusatory finger at her two schoolmistresses, Karen and Martha, saying she has seen them locked in an illicit embrace (though we witness no lesbian kisses).

Karen and Martha (Knightley and Elisabeth Moss from Mad Men) have invested everything in their homely, clapboard establishment for boarding girls. Yet Mary (Bryony Hannah) is believed by her indulgent grandmother (Ellen Burstyn) and by the hidebound community when a classmate backs up her claims. The stigmatised teachers threaten a slander trial and the local doctor – Karen's fiancé (Tobias Menzies) – insists he will stand by them. Little lies are pervasive, however, sucking everybody into an abyss of uncertainty.

Though almost certainly heading for a Broadway transfer, Ian Rickson's production has a few weak points (and the NT's memorable 1994 premiere to rival). It gets off to a slightly slow, over-choreographed start with a bunch of schoolgirls goggling at an erotic novel. Carol Kane does not get many laughs as Martha's parasitical, chiffon-twirling aunt. And the play's final Act has just one narrative twist too many.

The evening is mostly riveting, nonetheless. Hellman combined her radical condemnation of lesbo-phobia with a chain of deftly crafted plot developments which – while old-school in a way – keep you hair-raisingly in thrall.

One might cynically wonder if Knightley and Moss were unimaginative commercial casting (faintly resembling Audrey Hepburn and Shirley MacLaine from the 1961 film version). Knightley isn't half bad, though, with initial hands-on gentleness, later spiralling into despair and rage. Moss is electrifying and psychologically complex, with smothered jealousy. Menzies is on top form as the almost heroically stalwart Dr Cardin. And Hannah's scrawny Mary is terrifically unsettling, with the body language of a screwed-up teenage boy: a hand slung, in frustration, up the back of the neck, then darting out viciously, or grasping desperately for affection.

In Sir David Hare's outstanding early play Plenty (from 1978), Susan Traherne is a would-be free spirit ahead of her time. Her peregrinations and changing character are set against a background of major political events, including the Suez Crisis. Hare cuts back and forth, giving us fragmentary glimpses of Susan in the Forties, Fifties and early Sixties.

In Thea Sharrock's superb studio production – opening the Crucible Theatre's Hare Season – we see Hattie Morahan's Susan terrified yet liberated by living dangerously in the Second World War. Working undercover in occupied France, she has an unforgettably romantic brush with a stranger, codenamed Lazar (David Bark-Jones). Then, hoping for a new dawn, she hangs out in London with Kirsty Bushell's Alice, a louche boho in gender-bending pinstripes.

Nonetheless, post-war life disappoints (echoing Terence Rattigan's After the Dance). Susan sells out and marries a rising diplomat called Brock (Edward Bennett). Then unable to brook the role of rich, demure hostess, she goes increasingly crazy, launching a verbal blitzkrieg over ambassadorial cocktails, and leaving Brock's life in shards.

Compared with some of Hare's more clunking epics, Plenty combines the political and the personal with a structurally light touch and some wonderfully droll dialogue. Sharrock's thrust-stage production is poignantly intimate, Morahan brillinantly mercurial, and Bruce Alexander priceless as Sir Leonard, Brock's winded but still punctiliously clipped superior. Meanwhile the production quietly brings out – through Brock's self-sacrificing marital devotion – the famously left-wing playwright's compassion for essentially decent, conservative types, and his mistrust of scornful proto-feminists.

Susan's romantic and wealth-ditching ideals prove semi-deluded while, in The Heretic, Juliet Stevenson's Dr Cassell argues that Anthropogenic Global Warming is our era's dubious orthodoxy. AGW is, she holds, an empirically unproven popular religion, greed having sparked guilt.

In Richard Bean's new, provocative seriocomedy, her Climate Change Sceptic or CCS stance infuriates her funding-driven university boss and ex-lover (James Fleet). She simultaneously receives death threats from eco-guerillas who've found out her home address, as has Johnny Flynn's Ben, a twitchy student apparently taken with Diane's bolshy daughter.

Accompanied by whiteboard graphs demonstrating how statistics can be manipulated to look scary, the CCS arguments are certainly thought-provoking. However, they're sidelined by a romcom happy ending, and The Heretic feels generically heterogeneous. Still, it's quite frequently hilarious and, when death becomes an immediate danger, the suspense and raw emotion is startling.

'The Children's Hour' (0844 871 7622) booking to 2 Apr; 'Plenty' (0114-249 6000) to 26 Feb; 'The Heretic' (020-7565 5000) to 19 Mar

Next Week:

Kate Bassett takes a peek at Alan Ayckbourn's Snake in the Grass at new London venue, The Print Room

Theatre Choice

Bruce Norris's sharp US satire Clybourne Park dealing with race relations over half a century, now at Wyndham's in the West End (to 7 May). Also in London, Becky Shaw is a sardonic US sitcom, slick but with an edge of menace, at the Almeida (to 5 Mar). If farce is your bag, Tom Hollander is hilarious, doubling as uptight gent and drunken porter in Feydeau's A Flea in Her Ear (Old Vic to 5 Mar).