The Sugar Syndrome, Royal Court Upstairs, London<br></br>Rabbit, Lyric Hammersmith, London<br></br>Thoroughly Modern Millie , Shaftesbury, London

The darkest recesses of the chatroom
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I'm starting to wonder if this is, officially, Teenage Dysfunction Month. Theatrically, you aren't cutting edge unless you're portraying messed-up youngsters, preferably with enraging and enraged parents. New playwrights are, in fact, tapping into a potentially rich dramatic vein here, dealing with everyday yet intensely unhappy adolescents who are determined to live dangerously in very contemporary ways. The Sugar Syndrome is a gripping case in point. In this outstanding debut by writer Lucy Prebble, the first voice you hear is electronic. Dani (Stephanie Leonidas) is sitting in her parents' darkened living room, the glow from her laptop illuminating her gamine face. She dials up the net and is welcomed by Chatarama - "Chatrooms by location. Enter postcode." After all the recent news items concerning internet predators, the audience tenses instantly, virtually clutching their heads as Dani grins, types swiftly, and announces herself as: "Seventeen. Female. Wants it." There's a flurry of bings and six messages.

Lewis (Will Ash) is lying on his rumpled single mattress. He's 22, scruffy and has clearly been waiting for her. Dani has only to pick up her school rucksack and cross the stage to be in Lewis's bedsit. She is also encouraging a second chatroom friend: Tim, a middle-aged paedophile who thinks he's conversing with an 11-year-old boy. They arrange to meet on the swings in the park.

This could all have gone terribly wrong, not just for Dani but as a piece of theatre. The Sugar Syndrome is obviously loaded with issues which are hard to separate from media headlines and a sense of melodrama. The set-up feels a tad schematic too because Dani and Tim have both been "inside" for compulsive abnormal habits; she has been packed off to a rehab centre for anorexia and bulimia, kicking against her problem-repressing mum and her wealthy but absent dad.

Nevertheless, Prebble avoids the predictable horror story while creating tense moments en route, not least when the jilted Lewis turns up at Tim's council flat. Prebble also writes winningly unaffected dialogue in the main, shot through with droll quips. Some might complain this is excessively liberal fare, sympathising with the paedophile's predicament, but it is thought-provoking, touching and not simplistic. Prebble questions clear-cut definitions of who is criminal, psychotic or innocent, making you wonder to what degree nice guys or girls can turn nasty. Marianne Elliott's fine cast realise they don't need to highlight the flashes of menace, producing emotionally complex, tender and worrying scenes. Though Leonidas in infantile mode seems a tad exaggerated, this is an extremely assured young actress who should go far, and Andrew Woodall is quietly excellent as the wry, weary Tim. Jonathan Fensom's set design also shuns the hi-tech screens one might expect, cleverly creating a small neighbourhood with one sliding diagonal wall.

Frantic Assembly's latest touring production, Rabbit, offers some striking parallels. In this family drama, written by Australian twentysomething Brendan Cowell, Madeline Cave is a rich kid belligerently playing the lowlife hussy on the hip-hop scene. Like Dani, she can't stand the mindless wittering of her mother and is bitterly angry at her wealthy but inattentive dad. Her boyfriend, Spin, is a junkie; she introduces him to her parents at their chic holiday home just before she learns her father is dying of cancer. In the central family row, ferocious adolescent hatred is aired, but a move is made towards forgiving gentleness. Sam Crane's stringy, socially flailing Spin also has some very funny lines as he strives to be cool. But more often Cowell's dialogue is clumsily stylised and the second half, where everyone goes crazy, is boring and superficial. Frantic's co-directors Scott Graham and Steven Hoggett soup up the script with their trademark hip choreography: Helen Heaslip's Madeline is permanently "clubbing", lewdly dancing around her parents.

That hovers interestingly between realism and expressionism and strongly conveys her arrogance and erotic charge. The staccato pacing and spinning of Mr and Mrs Cave (David Sibley and Susan Kyd) is also a sharply punctuated study of frustration. Still, ultimately this piece is a drag and, given the company's young fan base, one can't help feeling the sexily choreographed heroin trip is deeply irresponsible, with Crane's groping Heaslip's fit bod while she flies in ecstasy above his head.

The trouble with Thoroughly Modern Millie is that this big West End musical proves to be vacuous and old-hat. Some will recall the 1967 film of this rom-com where the titular small-town girl comes to Manhattan as a gold-digging secretary who plans to marry her boss, Mr Graydon, but falls for Jimmy Smith, the guy in the street who is, secretly, stinking rich as well.

This stage version of Robert Morris's story, scored by Jeanine Tesori, opened on Broadway last year and won a Tony. Gee, it must have been a thin year. Dick Scanlon's lyrics are feeble and the 1920s setting is a series of clichés from its cloche hats down to its bathetic moral conclusion ("Follow your heart"). To be fair, some of the numbers are fun. Rob Ashford's choreography can be playfully inspired, and the chorus line are tip-top movers; the typists' routine is particularly natty. Though most of Tisori's tunes are instantly forgettable, many are pleasant en route. Craig Urbani's square-jawed Graydon is irresistibly farcical, bursting into a mock-operatic passion. Maureen Lipman is having a blast as the wicked, panto-style , faux-Chinese hotelier. However, any collective bounce is spoilt by Amanda Holden's rigidly nervous performance as Millie. Living dangerously clearly isn't her forte. She dances the Charleston with all the brio of a rabbit caught in headlights. Aw, honey, you can go back to Kansas if you want.

'The Sugar Syndrome': Royal Court, London SW1 (020 7565 5000), to 15 Nov; 'Rabbit': touring (info: 020 7228 8885), to 28 Nov; 'Thoroughly Modern Millie': Shaftesbury, London WC2 (020 7379 5399), booking to 22 Jan, 2004