This excellent production manages to combine such explosive themes as power, violence, notions of guilt, psychological torture and the competing loyalties of faith and family with a persuasive and surprising theatricality.

Janet Steel, a director who has previously worked with both Kali Theatre and the Birmingham Repertory Theatre, has managed to harness the high production values of the rep to bring to life the inventive scope that Kali allows its writers. The production notes state that Kali aims to support women writers of a South Asian background 'challenge our perceptions through thought-provoking theatre' and it certainly achieves this, the only disappointment being that women are entirely missing from the onstage action. Steel, too, is a director unafraid to explore difficult themes; she directed the Birmingham Rep's production of Bezhti (Dishonour) in 2004, only for it to be forced to close on its third night following violent protests in Birmingham.

Convicted of the manslaughter of a teenage boy during an exorcism, and serving out his sentence in an isolated wing of a prison, we watch as Mustafa struggles to contain and destroy the evil 'Djinn' he believes has possessed him. The Djinn, which Mustafa is certain caused the boy's death, creates violence and a profound sense of uncertainty and jeopardy wherever it goes. It infects all of the characters, and they all fall prey to its insidious power with shocking results.

Whether this Djinn is simply a violent extension to Munir Khairdin's wonderfully self-contained Mustafa or an alien force that possesses him and renders him unanswerable for his actions is the central question. We are split between sympathy for the battle he rages to defeat the Djinn and the evident fact that by the standards of today he is guilty of an appalling crime.

The fine line between victimhood and culpability is explored through the eyes of Mustafa's solicitor-brother, Shabir, who is torn between love for his brother and his acceptance that Mustafa must be guilty. Shabir is the mouthpiece of the secular, incredulous world and its laws, who despite his sympathy cannot forgive Mustafa his crime. In a confrontation towards the end of the play, the two devastated brothers attempt to understand each other in a touching and sensitive scene in which Mustafa exclaims devastatingly 'I was frightened!' and in which Shabir is forced to admit, too late, that he understands. They are caught in a struggle to find an alternative path to the black and white world of morality in which the only two options available are guilt and innocence.

It would be easy for this play to fall into generalised notions of the prejudices of secular guards and religious prisoners alike, but happily the writing and performances avoid this. Paul McCleary's portrayal of Len, a jaded prison guard, is nicely pitched between uncertainty and a desire to do the right thing, in spite of his failings. It is an affectingly human portrayal and to Naylah Ahmed's credit that she is able to explore seemingly stock characters without ever denying them the right to change their minds and surprise us.

Despite its themes, humour is never far away in this production and provides light relief and depth to the characters. The two guards provide most of the laughs, which span from dark irony to the plain silly as they search Mustafa's cell for signs of food and ponder the meaning of a chalk circle he has drawn on the floor. Len reassures Mustafa that he respects religion, earnestly saying 'You have laws of Islam, we have the prison rules'. It is a bleak humour but welcome nonetheless.

The dramatic shifts produced as the characters slip between themselves and the Djinn are both disturbing and suspenseful, greatly assisted by the sleight of hand achieved by the stage illusionist Richard Pinner and the lighting and sound design, all of which are imaginative and effective, adding greatly to the general sense of menacing expectancy which permeates the play.

Colin Falconer's set is excellent. Mustafa's cell is overlooked by a sinister network of caged corridors which give the small upstairs studio at the Soho Theatre a surprising sense of space and depth; prison guards seem to appear from nowhere while the clever lighting allows other characters to linger, poignantly catching or avoiding Mustafa's eye as they arrive and leave.

Overall we are left with a sense of the impossibility of understanding the motives and actions even of the people we love. It speaks profoundly of the isolation we feel when doubt is planted in our minds and of how the horrors of half dreams can rise to become the very things that destroy us. A sensitive and shocking piece of work which is likely to find more praise as it continues its tour.

Mustafa, at Soho TheatreSophie Lieven reviews Mustafa at the Soho Theatre.4