God of Carnage, Gielgud, London<br/>Never So Good, Lyttelton, London

The law of the playground rules as the grown-ups take the gloves off: Projectile vomiting, bilious sarcasm and a real-life power cut &ndash; only the dodgy philosophy mars a perfect farce

'I feel like showing myself in a horrible light!" barks Ken Stott's infuriated Michel, refusing to keep up politically correct appearances in Yasmina Reza's darkening comedy, God of Carnage. And to think it all started so well. As the curtain rises in Matthew Warchus's cast-to-the-hilt premiere – starring Ralph Fiennes, Tamsin Greig, Janet McTeer and Stott – two middle-class couples are perched around a coffee table. The room is pleasantly furnished: a comfy sofa, nicely arranged tulips, a cultured stash of fine-art books. Hospitality is dispensed in the form of home-made fruit tart, and small talk is being exchanged – slightly awkwardly.

This get-together is really the first step in a parental peace-and-reconciliation process. Michel and Véronique Vallon's little boy has been hit in the park by Alain and Annette's son, wielding a stick. But the grown-ups have decided to be civilised about this, to agree on reasonable damages and see that the kids patch up their differences.

Ha, ha. Underneath, they are all galled and spoiling for a fight – not just hosts vs visitors but also spouse vs exasperating spouse. Creating moment-by-moment tension more subtly than the blood-red walls of Mark Thompson's set, the veneer of niceties begins to crack, little jets of bilious sarcasm start shooting across the conversation and then all hell breaks loose.

This contemporary French farce, vibrantly translated by Christopher Hampton, is really a black comedy of degenerating manners. By the end, McTeer's Véronique – who likes to think of herself as a decent liberal lefty – is roaring like a lioness, necking neat rum and brandishing a chair. Casting off her nervous manner, Greig's chic Annette has gone wild too, screeching like a gleeful witch as she deranges the flowers – shredded stems spraying through the air like tracer fire.

Meanwhile, Fiennes' bruisingly rude Alain has been treating the Vallons' living room as his office, constantly taking calls. An arrogant lawyer, he is audibly advising a pharmaceutical company to cover up a scandal, displaying lucre-led ethics little better than a criminal's. When Greig finally snaps and trashes his precious mobile, he – most gratifyingly – crumples like a puppet with its strings cut.

On press night, unfortunately, God of Carnage proved to be a darkening comedy in a literal sense. Shaftesbury Avenue was hit by a partial power cut and the performance ground to a halt. Then the cast had to carry on under a few feeble, atmosphere-wrecking working lamps. "I feel like showing myself in a horrible light!" You can say that again. Unsurprisingly, initially, the cast struggled to crank the energy back into the dramatic conflict and, in the process, what caught one's ear was the banality of Reza's philosophising.

This dramatist's seriocomic writing can, indeed, be rather like a dodgy electrical connection: it sporadically falters, suddenly looking over-schematic, its grander themes crassly spelt out. Though it may seem fitting, from the writer of Art, to have Véronique and Annette suddenly pore over the paintings of Francis Bacon and ruminate on their cruelty, majesty and chaos, it's irritatingly self-conscious and feels superficially appended.

Nonetheless this is a fierce and very funny satire. It's like Who's Afraid of Virginia Woolf? crossed with Alan Ayckbourn and it will surely be a big hit. Reza certainly strikes a nerve, exploring how belligerence isn't just the nub of international conflicts but rife in our own homes, and how the apparently cultured and the well-to-do – and women – can be the worst aggressors. Les bourgeois really are épatants.

The evening offers unforgettably, appallingly hilarious scenes, not least Greig projectile vomiting over the art books and Stott stoically applying a hairdryer to the bile-dampened pages. Warchus's ensemble doesn't have a weak link and Greig and Fiennes are particularly sharp on the tell-tale body language: Greig's fingers neurotically dusting her elegant knee, Fiennes leaning forward to shovel the fruit tart into his mouth with all the finesse of a jackal. I'm not sure that President Sarkozy, in the week of his state visit, will thank his compatriot for this portrait of diplomacy going to the dogs and entente cordiale being blown out of the water. However, you know you're in the presence of true theatrical luminaries when they can do incandesce without the help of EDF Energy (French-owned).

There are some blinding wartime explosions over at the National Theatre in Never So Good, Howard Brenton's new bio-drama about Harold Macmillan. A moustachioed, mild-mannered Jeremy Irons addresses us as the British PM in old age, with his 20-something self played by lanky Pip Carter. They step back in time to the injuries Macmillan sustained on the Somme. Then we move on through his early loyalty to Churchill, the Suez Crisis, his quiet ousting of the amphetamine-guzzling Anthony Eden, and then his own short time in power, pulled down by Peter Cook's Beyond the Fringe impersonation and the Profumo affair.

I spent much of the first half of Howard Davies' production thinking this was Not Very Good. Anna Carteret is slightly wooden as Macmillan's domineering mother. Anna Chancellor seems initially strained as his passion-deprived adulterous wife. The bouts of choreography (by Lynne Page) are tiresomely extended, making one wonder if the NT thinks a chorus line doing the Charleston or the Twist is needed to jolly up political history.

The most astonishing fact is that this sympathetic portrait of the old Conservative is by Brenton, the famously excoriating left-winger. I too mellowed, given time. Davies has produced a compassionate portrait of considerable complexity here. The second half of the evening becomes gently touching, as well as psychologically intriguing in its mix of moral decency and political machinations.

Irons' performance is beautifully subtle – wryly humorous and quietly wounded – and you learn some illuminating lessons from history along the way, with barbed relevance emerging about badly planned occupations and destabilising relations with the Middle East.

'God of Carnage' (0870 040 0046) to 14 June; 'Never So Good' (020-7452 3000) to 24 May

Need to know

Yasmina Reza, the chic French actress-turned-playwright, now 48, first made UK headlines in 1996 when her Parisian comedy, 'Art' – about three pals who quarrel over an all-white painting – became a West End hit, backed by Sean Connery and directed by Matthew Warchus. Warchus then scored a hit at the NT in 2000 with 'Life x 3', starring Mark Rylance: more souring relations in designer living- rooms. Turning to political biography, Reza caused a stir in France last year with her close-up account of Nicolas Sarkozy's election campaign.