WHOEVER it was at the National who persuaded a French film star to play a Scots queen in an English translation of a German tragedy should step forward. They deserve a round of applause. The compelling reason to go and see Schiller's Mary Stuart - aside from the fact that it is a magnificent play - is an astonishing performance by Isabelle Huppert.

It is perfectly reasonable, of course, for an actress playing Mary to speak with a French accent, because the young Queen grew up in the French court. But there's more to it than that. From the first scene, when Huppert defiantly argues with Lord Burleigh (Paul Jesson), swishing and turning, jabbing her finger at him and demanding to see the evidence, you see that casting a French actress has rich dramatic potential. The fact that Huppert is speaking a foreign language heightens her isolation and sense of entrapment. She has to work all the harder.

Of course, the downside is that we do too. Sometimes her thoughts are lost in misplaced emphases and slurred phrasing. (Huppert is not helped by having to compete at particularly intense moments with an underscore.) But she turns the challenge to her advantage. You sense her actively searching for the right word to describe a courtier, or making that extra effort to hit the "h" in "heart", "haunted" or "hatred". The urgency to communicate her point of view is overwhelming. Both Huppert and Mary strive to assert themselves in another culture.

Huppert has more success than Mary. A tiny figure, she dominates each scene she is in with an undaunted emotional physicality: whether skittishly dancing like a young girl round her loving nurse (Gillian Barge) or fainting erotically into the arms of Leicester (Tim Pigott-Smith) on her way to the scaffold. One of the most dramatic moments I've seen is when Huppert meets Elizabeth (Anna Massey) for the first time. "God help me," she observes, fatefully, "that is a face that hasn't got a heart." She sinks to her knees and waits, and waits, arm extended, for the sour and chilly Queen to raise her back up to her proper station. Elizabeth never does. Huppert fills this moment - like so many - with a desperate pride and vulnerability.

Elsewhere, Howard Davies's production displays an ironic sensi- bility that resists embracing the tragedy with quite Huppert's full-bloodedness. It entertains us while undercutting the momentousness of events. Jeremy Sams's deft, witty translation supports this, with its taste for theatrical metaphors and its relaxed attitude to modern phrases ("diplomatic immunity", "league of nations"). The costumes slip between periods, with Elizabethan dresses for the monarchs and silk frockcoats and trousers for the lords. Though Anna Massey is sharply impressive as Elizabeth, there is a hint of caricature: we revel in her scheming, petulant nature as she raspingly upbraids the Secretary of State for doing exactly what she has asked him to do, while keeping our distance when she bares her soul. Similarly, Pigott-Smith's double-dealing Leicester has foppishly witty flourishes, but his powerful monologue on witnessing Mary's journey to the scaffold sinks beneath a compendium of director's cliches. (There's red lighting, smoke, walls closing in, an echo machine and an underscore: the speech on its own would have been just fine.) None of this, however, can dimHuppert's electrifying performance. If this paper wasn't republican, I'd call her majestic.

The Soldier's Song, a first play by Bryan James Ryder, comes over not as one but as two first plays. You think you know where you are when a bloke says to his wife, "I'm my own man. Independent," closes the door, waits a beat, then opens the door and reappears asking: "Where's my clean shirt?" It's gag land. Looking round the living-room set, there are other reassuring signs: chintzy sofa, mahogany dresser and decorative plates hanging off the picture rail. Then there's the family: mum, dad and two grown-up kids. The son has had a fling with a friend of his sister's, and the friend is coming round to tea.

Yet The Soldier's Song is nothing like a domestic comedy: it's tense, revelatory and absorbing. The house in question is off the Falls Road. The McManuses are a West Belfast Catholic family. It's the eve of the Ceasefire. Unknown to his parents, son Eamon (Billy Carter) is in the IRA, and about to do a "job". He can't say no to it. If he does, as his friend and IRA colleague (Andy Robb) tells him, he's booking himself "an appointment with a ditch in South Armagh". The Soldier's Song offers us, simultaneously, something cosy and recognisable - a dramatic setting and plot devices that could serve any number of B-dramas - and something raw, topical and bleak.

Ryder grew up on the Falls Road, and, even without the terrorist plot- line, it is shocking to see how every aspect of domestic life is shaped by the presence of the British Army and the IRA. Ryder depicts, accurately and wittily, the range of attitudes within the family: the feckless, bigoted father (Colin Tarrant) who won't allow a Protestant in the house; the tolerant, hard-working mother (Anne Carroll), who is also the breadwinner; and the understandably selfish daughter (Sarah Howe), a university student who can't wait to leave "this slum" behind. Here is the Catholic community presented with a refreshingly uninhibited eye. Witness the wheedling neighbour, May (Mary MacLeod), who complains that the only time she can afford to buy curtains is when "the Provos firebomb the shops". It's the kind of observation only a writer who is properly in residence can make.

Richard O'Brien, creator of The Rocky Horror Show and (as Reckless Rick) one-time presenter of Channel 4's The Crystal Maze, plays another self-consciously wacky figure in his latest show, Disgracefully Yours. As Mephistopheles Smith, O'Brien appears in tails, with little horns on his bald head, a tail and cleft hooves. Tall, thin and remarkably ageless (for 54) he welcomes us, with a vulpine grin, to the Cafe Inferno, plc. Here he has the hard task of convincing a sedate West End audience that they are at a party ("You give great hedonism," he flatters us, with one of his dafter puns). Somehow you feel that O'Brien has been to parties that are a lot better than this one. Disgracefully Yours is more a cabaret than a West End show: a rock gig, with campy gags centred on a tired theme. Even Chris Tarrant has a jingle about being the DJ from hell.

`Mary Stuart': Lyttelton, SE1 (0171 928 2252). `The Soldier's Song': Stratford East, E15 (0181 534 0310) to 13 April. `Disgracefully Yours': Comedy, SW1 (0171 369 1731) to 6 April.