When Carlos Santiago, a serial killer, is caught, Titus Winston is appointed to prove that he is insane and, therefore, not responsible for the murders he undoubtedly committed.
Santiago has no memory of the crimes, but when questioned, he changes into another personality, claiming to be Satan. Winston and prison chaplain James Dawson interview Santiago/Satan, who discusses the biblical events he has apparently caused.
Santiago indeed appears to be two people and Titus and James must establish the truth. The effect on them is profound as Satan outlines his plan to take over the world. Their beliefs are tested to the limit. Is all the world's evil caused at Satan's wish? Is Santiago a well-informed, vicious murderer, hiding behind a fictional alter ego? Or worse, is that terrifying alter ego real? Is the power of prayer enough to prevent the next stage of Satan's plan being carried out? Will Satan win them over to his side, or God to his?