'Some things can't be conversational," says King James I to his English queen, Joan, in the first of Rona Munro's James Plays, a potentially important Scottish history trilogy for the National Theatre of Scotland, the National Theatre of Great Britain and the Edinburgh International Festival.

It's a line which slaps one in the face with its unintended irony.

Munro's seven-and-a-half hour epic is so overwhelmingly banal in its vernacular, so light in its intellectual touch and so weak in its dramatic shape that, by its stuttering conclusion, it feels like the biggest missed opportunity in the history of Scottish theatre.

It comes as no revelation that medieval Scotland was an unstable and brutal society in which the monarch was constantly under threat from within and without his kingdom. The challenge of such an ambitious project as this is to make theatrical drama out of the greed, perfidy and violence.

Great imaginative dramatists, such as Shakespeare, Schiller and Howard Barker, take historical moments and shape them into brilliant, robust-yet-poetic art works. By stark contrast, Munro's characters speak, for the most part, in argots of straightforward statement or, worse, the puerile abuse of adolescents in 21st-century school playgrounds; the expletive-laden conflict between James II and his erstwhile friend William Douglas is a particularly depressing case in point.

Moreover, in its apparent quest to be (in that seemingly democratic, yet so often artistically destructive term) "accessible", the trilogy is sprinkled with easily anticipated jokes about the Scottish weather and the supposed Scottish national character. Where one longs for genuine wit and theatrical gravitas, one gets, instead, ingratiating frivolity.

There are some attempts at poetry, to be fair. The proud, avaricious and rebellious Isabella Stewart (played by Blythe Duff, the outstanding actor of the trilogy) speaks in a dark, angry tongue, but even her metaphors tend to founder on their obviousness.

Director Laurie Sansom's often histrionic production fails in the surely impossible task of making a coherent drama out of Munro's scripts. Yet, to the ordinariness of the written speech and the two-dimensionality of the characterisations, his staging adds considerable variability in acting. Some young actors - notably Sarah Higgins as Meg, the no-nonsense lady of the court - rise to the challenge. Others are clearly out of their depth.

However, the work's greatest sin against the theatre is its often meandering, shapeless form - James II: Day Of The Innocents, in particular, has all the structure of a badly made blancmange. The history of the Scottish Middle Ages may be chaotic; what it is not, is boring. That Munro's disposable and unmemorable trilogy contrives to be tedious is its ultimate disgrace.

While the James Plays are, on account of their high profile, playing to undeservedly large audiences, Mundana Companhia of Brazil are currently receiving scandalously small houses for one of the most beautiful productions of this year's Fringe. A three-hour adaptation of Chekhov's novella The Duel, it brings a gorgeously Brazilian energy and imagination to the great Russian's tale of moral and erotic conflict in a declining Black Sea town.

A universally brilliant cast is led by the superb actors Aury Porto (as the dissolute Laevsky, recently escaped to the Caucasus from St Petersburg) and Camila Pitanga (playing Nadyezhda, Laevky's lover, who is married to another man). What unfolds is, by turns, a deeply tragic and sensual journey through two parallel narratives: the painfully distorted love affair of Laevsky and Nadya, and the conflict between the homicidally contemptuous Social Darwinist Von Koren and the Bohemian Laevsky.

The stories are often revealed in startlingly imagistic terms. Director Georgette Fadel employs deceptively simple props, such as a versatile piece of clothing which can swallow a person entirely, or a huge cellophane bubble which absorbs people in touching metaphor. The novella's sense of perilous sexual possibilities and growing physical danger is enhanced marvellously by a use of music and dance which is absolutely Brazilian.

The unatmospheric EICC is the wrong venue for this excellent piece. In fact, it deserves, more than most of the shows I have seen in this year's EIF drama programme, to be staged by the International Festival. No theatre lover who is in Edinburgh for the festival should miss this exquisite production.

Nor should anyone miss Return To The Voice, the latest extraordinary work from ever-impressive Polish theatre company Song Of The Goat. Thanks to director Grzegorz Bral's collaboration with Rupert Thomson, visionary artistic director of the Summerhall venue, the Goats have worked with many practitioners and experts in the fields of traditional and ancient Scottish and Gaelic musics to create a work of breathtaking power and beauty.

The international company (which is based in the Polish city of Wroclaw) is strongly rooted in the methods of the great Polish theatre master Jerzy Grotowski. It is quite remarkable to see and hear these methods combined with the cadences and tonalities of, for example, the Gaelic music of the Highlands and Western Isles.

This show is neither an attempt to recreate centuries-old songs nor a mere choral concert. Rather, it is a hybrid of the new and the ancient. At moments, as the Goats' remarkable performers engage in gestures of pain, desire, love and grace, one senses that the most seminal elements of theatre and human performance have come together with the oldest, most endangered sounds from Scotland's musical heritage. The result of this deeply considered, brilliantly wrought synthesis is a work of profound, soul shuddering beauty.