Posh, Royal Court Downstairs, London<br/>Hair, Gielgud, London<br/>The Empire, Royal Court Upstairs, London

The Royal Court is doing its darndest to sabotage the Conservatives' election campaign. That's what it looks like anyway, because Posh – Laura Wade's new main-house play – is a fictionalised group portrait of something not that far from the Bullingdon Club.

If anyone needs reminding, that's the dining club of super-rich and aristocratic Oxford University chaps whose longstanding custom it is to smash up local restaurants, and then escape trouble by throwing money at the gobsmacked staff. David Cameron, George Osborne and Boris Johnson are old Bullingdon boys.

In Lyndsey Turner's superbly cast ensemble production, nine young toffs and their feckless president, Tom Mison's James, descend on a gastro pub in the Oxfordshire countryside. They hire a private room – ox-blood red with antlers on the wall – and dress up in archaic tail-coated uniforms, as in those 1980s photographs of Cameron and co. Except this bunch call themselves the Riot Club and, since they're using an iPhone to check that a call girl is on her way, the setting must be now. Thus Wade keeps one step clear of a potentially libellous biodrama, while exploring the broader possibilities of a political and economic allegory.

Her point is partly plus ça change. In a darkly satirical vein, these arrogant twerps who regard top City and parliamentary posts as their birthright build up to their act of shameless vandalism, while preserving the Club's ludicrous traditions. They bray the National Anthem, make endless toasts to long-dead members, and no one is allowed to leave the room. Leaving the room is a club offence, so sick bags are provided.

What's hair-raising, as well as comical, is how coarsely bigoted these supposedly well-bred chaps are behind closed doors, slipping on the mask of abstemious decency whenever they have to placate the landlord, Daniel Ryan's burly Chris. There's a chilling trace of Patrick Hamilton's 1920s thriller Rope in these young gents' hidden brutality.

The tension mounts as they indulge in leadership in-fighting (a touch of New Labour there?) and as the masks begin to slip. This could all end in bloodshed. Leo Bill's drunken, vituperative Alistair rails against small businessmen – like this landlord – for wrecking the financial status quo, and his chums turn their attention to the waitress, the landlord's not entirely obliging daughter, Rachel (Fiona Button).

More intellectual brilliance would have been welcome. The political arguments are somewhat fuzzy even when Alistair is sober, and there is a whiff of demonising melodrama about the close, when he is conspiratorially recruited as future PM material. If there is a stand-out performance, it's David Dawson as the fey Hugo, with his feverish, glittering grin. However, everyone is superb: this is an array of young acting talent to rival that of The History Boys.

The hippy youths in Hair, transferring to the West End from Broadway in a buoyant revival, certainly wouldn't buy David Cameron's new proposal, that people's moral decency will really improve if you bribe them (three quid a week) to get married.

Unfortunately, everybody's so stoned in this vintage make-love-not-war rock musical (by Gerome Ragni, James Rado and composer Galt MacDermot) that they can hardly string a countercultural sentence together. There's a lot of mumbojumbo sung about the sign of Aquarius.

Yet this show isn't completely dumb. It spots the emotional fracture lines opening up in the naïve concept of free love, even while Diane Paulus's cast – a swarm of colourful scruffs – whirl and writhe in mimed orgies, all with charmingly humorous gusto (choreographed by Karole Armitage). The famous scene where everyone gets naked which made British theatre history in 1967 – the day after stage censorship ended – has lost all power to shock. It now seems sweetly innocent, over in a flash.

The characters' spirit of political protest, principally against fighting in Vietnam, naturally has more trenchant reverberations today. Gavin Creel's Claude, the shy guy who daren't burn his call-up papers, ends up laid out for a military funeral.

Producer Cameron Mackintosh has certainly scored a hit, importing the New York Public Theater's cast wholesale. Caissie Levy's Sheila and Sasha Allen's Dione resoundingly belt out the funky and frisky numbers "I Believe in Love" and "White Boys". Will Swenson is terrifically wolfish as the oversexed Berger, climbing into the stalls and teasingly ruffling anyone he fancies. And there's a lovely democratic party at the end when anyone who wants to can pile onstage and sing along to "Let the Sunshine In". A tiny white-haired granny was high-kicking with Berger as I left.

Back at the Royal Court, in the tiny Theatre Upstairs, The Empire transports you to the British Army's on-going struggles in Afghanistan. DC Moore's outstanding and superbly acted four-hander is set in a rubble-strewn military shack in the baking heat. Joe Armstrong's sweat-drenched Gary is a laddy, white corporal, initially joshing with a morosely taciturn, local soldier-boy (Josef Altin), whom he calls Paddy. Things take a far uglier turn, however, when Nav Sidhu's Zia is dragged in from a nearby battlefield, robed in a traditional kameez. He's obviously a member of the Taliban, only it turns out he's a motormouthed Eastender. He says he was just out here visiting family when he got kidnapped. Tempers are fraying dangerously under the stress of attack, delayed helicopters, lost lives and suppressed despair.

Moore has a fantastically sharp and humorous ear for military slang. This is combined with director Mike Bradwell's quietly superb naturalistic detailing, from distant birdsong and buzzing flies to long, exhausted silences. When Gary and Zia both lose their rag and the uppercrust captain, Rufus Wright's Simon, tries to regain control, what's even more extraordinary is that this chamber piece becomes a scorching and far-reaching exposé of how the British empire's lingering class system, two-way racial hatred, and decadent capitalism have created a ticking bomb.

'Posh' (020-7565 5000) to 22 May; 'Hair' (020-7907 7071) to 8 Jan 2011; 'The Empire' (020 7565 5000) to 1 May

Next Week:

Kate Bassett tests the veracity of The Real Thing, Tom Stoppard's tangle of love, life and acting, revived at the Old Vic