To be, like Cain, the first-born child of the first man and woman on earth, is to suffer from a distinct shortage of inspiring role models. After all, why should you obey parents whose own primal disobedience has had such spectacularly catastrophic results: expulsion from Eden and the infliction of the painful penalties on all future generations? And why should you respect, still less worship, a God who imagines that it is perfectly just to punish people who weren't even born when the sin was committed.

Cain's proud discontent had a strong intuitive appeal for the untoadying, rebellious spirit of Lord Byron. His play on the subject was not intended for performance, being better suited (he maintained) to "mental theatre". Through his engrossing, finely judged RSC production in the Pit, John Barton, a director temperamentally drawn to plays that benefit from a little adapting, proves that Cain works on the stage as well as on the page. With Dennis Potter's Son of Man already in the repertoire, this means that the RSC now has an inspired pairing of works that provoked cries of blasphemy on their first appearance.

Barton's version begins with a group of people who look like Victorian- style New Age travellers. Seated round a camp fire, they listen as a white- bearded Griffith Jones reads the opening of Genesis. This scene gives way smoothly to Byron's drama, which begins with the already mutinous Cain refusing to join in family prayers and ends, after the murder of Abel, with his rejection by all his kin save his sister-wife Adah. The heart of the play consists, though, in a long theological discussion between Cain and Lucifer who, in Byron's account, is quite distinct from the serpent who tempted Eve in the Garden and therefore less compromised in his dealings with mankind. He takes the youth on a cosmic journey, the immensity of which is suggested here with magically simple effects - the protagonist, arms stretched upward, twisting in a shaft of light; the post-lapsarian cracks in the stone floor suddenly irradiated, turning earth to sky.

Marcus D'Amico's Cain has a captivating straightforwardness and integrity. This is no adolescent brat whinging that he never asked to be born, but a youth in genuine philosophic turmoil, who, you believe, would lay down his life to bring his brother back. Admirably, he refuses to be tricked into worshipping John Carlisle's Lucifer, a lordly figure in a pale-grey frock coat and cravat and he registers the oddness of Lucifer asking for this treatment, given his opposition to the tyranny of God. "I never/ As yet have bow'd unto my father's God/...Why should I bow to thee?" He also spots the chink in this being's armour. Lucifer has an intellectual sympathy for the young man to the extent of saying that, if he had been the Creator, he "would not have made thee what thou art", but Cain's questions about love put him on the defensive. "I pity thee who lovest what must perish," announces this fallen angel, to which Cain pointedly replies "And I thee, who lovest nothing".

As Adah, the adored sister-wife, Thusitha Jayasundera attractively embodies the case for simple piety and strong family ties. She stops you from feeling that the most public spirited course for Adam and Eve after the fall would have been practising vigilant contraception.

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