Henry IV, Donmar Warehouse, London<br/>Gone Missing, Gate, London<br/>M.A.D., Bush, London

As Magritte might have said: Ceci n'est pas un palais. The Donmar has turned into a medieval palace with minions in hessian tunics leaning against massive stone pillars, waiting for the entrance of the Holy Roman Emperor, Henry IV. However this is a show, and I don't just mean director Michael Grandage is staging another long-overlooked European classic. Written by Luigi Pirandello in 1922, Henry IV obsessively blurs the boundaries between our realities and our fictions.

As Magritte might have said: Ceci n'est pas un palais. The Donmar has turned into a medieval palace with minions in hessian tunics leaning against massive stone pillars, waiting for the entrance of the Holy Roman Emperor, Henry IV. However this is a show, and I don't just mean director Michael Grandage is staging another long-overlooked European classic. Written by Luigi Pirandello in 1922, Henry IV obsessively blurs the boundaries between our realities and our fictions.

The characters, we soon learn, are all acting out the fantasy of a rich, modern-age madman (played by Ian McDiarmid) who has insisted that he's the aforementioned 11th-century ruler ever since he was thrown from his horse in a fancy-dress cavalcade. Arguably, he was already crazy because his love for the society beauty, Matilda, was unrequited and she had been riding beside him costumed as Henry IV's enemy, Countess Matilda of Tuscany.

Now middle aged and wearing Armani, Francesca Annis' Matilda arrives at the faux palace seeking reconciliation or a cure. She has a psychiatrist in tow who thinks that "Henry" should be presented with both his lost romantic dream (or rather Matilda's lookalike daughter) and the actual older woman. The tragic irony is this shock-treatment is administered moments after "Henry" has told his minions that he has been sane for a while.

Pirandello's set-up initially seems rather strained and dull, especially since "Henry" has mothballed his real life and made himself history, necessitating an exposition of the Salian Dynasty and Pope Gregory VII. At other points, one might accuse this playwright of disguising scrambled philosophical lectures as drama. Yet fascinating questions are raised about madness and identity.

It must be said the calibre of Grandage's cast is uneven, unless the enterprise was even more Pirandellian than I realised and certain actors were just pretending to be second-rate. Cavils aside, Annis' mix of regret and posed vanity is spot-on, and David Yelland is droll as Matilda's suave, snide companion. Beyond that, this production has two towers of strength. McDiarmid's performance is mesmeric. Shuffling around in sackcloth rags, with his sunken cheeks smeared with red crucifixes, he looks like a martyred clown. But there's a dangerous glint in his eyes. The path he charts between authentic and feigned madness is deft and unnerving. His rage, contrition and sorrow at his wasted life are fierce, reminding you this play is shot through with real griefs because Pirandello was driven to consign his unbalanced, jealous wife to an asylum. At the same time, McDiarmid is an impish mock-lunatic. He is Hamlet with a white beard and this makes you long to see his King Lear too.

The other triumph is the new translation by Tom Stoppard. He knows all about playing games with reality and theatrics, overlaying past and present, and making philosophical lectures sparkle (see The Real Inspector Hound, Arcadia and Jumpers). He also makes Pirandello's dialogue seem fresh, lobbing in vigorous expletives while being more subtly poetic at other points. The madman's image of naked human beings in bed is curiously haunting, "our clothes hanging up, watching over us like ghosts".

The last thing you see in Gone Missing is six bespectacled and besuited performers silently taking off their jackets and leaving them hanging in mid-air. It's like Gilbert and George shifting into the melancholy of Joseph Beuys. This outstanding experimental piece by the young US troupe, The Civilians, alludes to the missing persons of 9/11 but only hints at that huge tragedy. Taking a tangential approach, they have interviewed New Yorkers about material items they have lost and, from that, they've woven together verbatim theatre and cabaret numbers. It's a startling combination executed with great panache, satirical acumen and covert tenderness. Bizarrely, you feel you've met real people though the cast mercurially swop roles. Yarns about mislaid Agnès B scarves and Gucci boots are interspersed with interviewees who can only think of other kinds of losses - their jobs, their loved ones, their minds. The piece also slips in some broader philosophical musings on the pain of remembering things past and on worlds elswhere. Witty and oddly comforting. Catch this while you can.

In David Eldridge's new semi-autobiographical play M.A.D. about a working-class Romford family, 29-year-old John has lost his father. Distraught, he says he wishes he'd let his dad leave his mum and him two decades back. The rest of the play is set in the Eighties, with the 10-year-old John caught in the crossfire between his parents. There are awkward patches: the child-actor has overlong speeches and the analogy between marital and world conflicts is overstated.

Nonetheless, M.A.D., directed by Hettie Macdonald, is an acute study of collateral emotional damage. Jo McInnes and Lee Ross are superb as John's fond but bitterly frustrated mother and his deeply wounded father. In particular, the central scene, where the boy overhears their argument about him and an affair, has a raw ferocity and accuracy that knocks you for six.

'Henry IV': Donmar, London WC2 (0870 060 6624), to 26 June & touring; 'Gone Missing': Gate, London W11 (020 7229 0706), to 22 May; 'M.A.D.': Bush, London W12 (020 7610 4224), to 22 May

k.bassett@independent.co.uk