When, on December 16 last year, Jyoti Singh Pandey, a 23-year-old medical student, was brutally gang-raped on a bus in New Delhi, the attack (from which she subsequently died) sent shock waves across the globe.

Even in a world in which one might almost be forgiven for becoming inured to what Nick Cave has called "routine atrocity", this crime distinguished itself in its horrifying savagery. Jyoti became known as Nirbhaya ("Fearless One" in Hindi) on account of her brave resistance to her attackers.

Now, in a collaboration between a group of Indian women and acclaimed South African dramatist Yael Farber (creator of last year's Edinburgh Fringe hit Mies Julie), the pain and outrage caused by the attack have grown into a resonating work of theatre.

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Five Indian women (three of whom are professional actors, two of whom are not), assisted by a male actor, weave into the agonised narrative based upon their own experiences of sexist violence: the casual groping of the daily bus journey in Delhi; sexual abuse as children; a disfiguring, misogynist burning; and gang-rape on a street in Chicago.

That the play carries a heavy moral authority is undeniable. However, the genius of this piece is that Farber and her cast have, rather than create another work of documentary drama, turned to the unique capacities of live theatre.

From the representation of the attack on Jyoti to the re-enactment of her funeral, and the five narratives in between, the work ties together a visceral anguish with a bleak, beautiful poetry; be it in language, movement, visual imagery, or sound and music.

Great theatre requires the intercession of the human imagination, and this piece combines the presentation of unbearable facts with ingenious aesthetics in a manner so brilliant that it is, surely, one of the most powerful and urgent pieces of human rights theatre ever made.

There is, surprisingly, little such power in Somnambules & The 7 Deadly Sins at Summerhall. A collaboration between Brighton-based physical theatre artist Yael Karavan and Tanya Khabarova of acclaimed Russian company Derevo, it is a meditation upon the seminal forces at play in human experience and how they manifest themselves in the modern world.

As Edinburgh Fringe audiences have experienced with Khabarova's work, visual theatre is capable of great beauty and metaphorical resonance. In its more abstract moments, Somnambules creates memorable images, such as Karavan, in a white dress, struggling against the trailing fetters placed upon her by Khabarova, who is attired as a malevolent, demonic nun (an image of Death which, alongside a skull and a prominently placed chessboard, seems to have been ripped shamelessly from Ingmar Bergman's great film The Seventh Seal).

The show's ethereal use of music, sound and lighting will be recognisable to fans of Derevo. However, such moments of stage poetry are too few, and are overwhelmed by a well-intentioned, but heavy-handed, political commentary.

From militarism to environmental destruction, the bête noires of contemporary liberalism are trotted out with a numbing banality. By the dreary end, the only surprise is that these accomplished artists could have created such a crude and misconceived piece of theatre.

There is another contemplation of human crisis in The Events, one of David Greig's latest plays, co-produced by the Actors Touring Company and, it seems, just about everyone in Europe. In a non-descript church hall, we find Claire (Neve McIntosh), lesbian minister for a liberal Christian denomination, attempting to understand the motivations of the unnamed Boy (one of the characters played by Rudi Dharmalingam) who massacred members of Claire's multi-ethnic choir.

There is a compelling truth in Claire's increasing obsessiveness and its impact on every aspect of her life. Cross-casting Dharmalingam, an Asian man, as both Claire's female partner and the white supremacist mass murderer (who resembles a cross between the Norwegian fascist killer Anders Behring Breivik and his British counterpart David Copeland) is a smart, if not quite original, move; the words of The Boy, in particular, take on a strange, inscrutable aspect which cleverly mirrors Claire's bewilderment.

McIntosh and Dharmalingam give captivating performances, but Greig has hamstrung his play with a theatrical device which deliberately but detrimentally, upsets the drama's momentum. The actors play in front of and through a real choir (The Hadley Court Singers from Haddington, East Lothian), who represent the white survivors of the atrocity. The choir makes thematic sense; not least when we see them being subjected to Claire's efforts at collective healing (through a shaman ritual).

However, with the best will in the world, while the singers have lovely voices, actors they are not.

The requirement that they deliver dialogue in a number of set pieces is painfully untheatrical.

Ultimately, in stark contrast to Pornography (Simon Stephens's play about the 7/7 London bombings, which premiered five years ago), Greig's offering simply deflates itself too often to carry any real dramatic charge.