He's tackled Holocaust revisionism and Abu Ghraib. Now, Errol Morris turns his attention to the case of Joyce McKinney. He tells B Ruby Rich why this could be his final documentaryAn innocent, beautiful girl from the Blue Ridge mountains, once crowned Miss Wyoming, becomes engaged to a young man, then flies to London to rescue him from the religious cult that has kidnapped and brainwashed him, sacrificing her virginity to make love with him in a desperate bid to restore his sanity.