That Constantine and Aiah run up against is dangerously solid, breakably real.

This world has none of the soft edges of the let's-pretend kind of fantasy set in some sorcery-ridden backwater. Williams's invention of plasm is on a par with his invention of the world of Caraqui and its contentious neighbors. In fact, she has developed into a remarkably likable character, still enthralled by her mentor but a far more reliable witness, whose portrayal of Constantine comes into sharper focus as the shadows darken. In ''City on Fire,'' their passion and their need for each other are as strong as ever, but Aiah has grown stronger too, and her eyes are opened wider. To see Constantine clearly through the layers of hero worship. Since the book was written from Aiah's perspective, it was hard In the earlier book, he was the adored mentor, she the devoted disciple. Where the sequel outshines the original is in its treatment of the relationship between Constantine and Aiah. Despite the references to plasm-wielders as magicians, plasm is the science of Constantine and Aiah's world, and its propertiesĪre as calculable, and as apt to get out of hand, as anything wrought by the technology of our own world. Is ''plasm,'' a force that accumulates in matter and can be directed by human desire. Once again, the issue is power - who wields it and for what end - and the worldly source of power

'City on Fire' describes what happens when Constantine and Aiah try to consolidate their victory in the city-state of Caraqui. Promised to renew a tired old world and usher in a vaguely defined utopia known as the New City. Together, they pulled off a revolution that (1995) introduced the yoked conspirators Constantine and Aiah - he of the keen mind, hard body and overpowering will she of the humble background, bleeding heart and unquenchable ambition. ITY ON FIRE (Harper Prism, $22), by Walter Jon Williams, is that rarest of entertainments, a sequel that improves on a successful predecessor.

At the City Limits of Fate By Michael Bishop.

Metropolitan by Walter Jon Williams