His memory has been erased. Hers was destroyed by a disease, and reconstructed with his help. His real name is David Avery, but he knows himself as Derec. Her name is Ariel Burgess.

Together they found Robot City and plumbed its mysteries. Derec, at peril to his life and in the throes of one of his mad father’s experiments, learned to master Robot City and its robots. Hordes of chemfets-microscopic robots-in his blood gave him a direct connection with the central computer.

During a brief idyll, Derec and Ariel lived normal lives on Aurora. But Derec’s final confrontation with his father had interrupted what the robots called the Migration Program-the program had not been canceled. Some robots had escaped from Robot City and had built new robot cities on new, uninhabited planets. Planets, at least, that were supposed to be uninhabited.

Supposed to be, but were not. Derec’s placid interlude was shattered by a distress call from one of the new robot cities, telling of an attack. Rushing to the scene without Ariel, he and Mandelbrot discovered that the attackers were beings who looked something like wolves-a race of intelligent wolves.

First, there was a meteor flashing through the sky. Then the strange one came, the metallic-looking one they called SilverSides, who never ate and wished only to protect the Kin and serve their wishes. It could only have been that SilverSides had been sent by the OldMother, ancestress and creator of the Kin. She had been sent to save them from the WalkingStones and the Hill of Stars they had built.

Not even SilverSides knows that she was a robot, cousin to the robots that were building a robot city on the Kin’s planet. She had been designed and built not by Dr. Avery, Derec’ s father, but by Dr. Janet Anastasi, Derec’s mother, who was running her own experiment in robotics.

SilverSides had been born shapeless, unformed, ready to imprint upon the first intelligent being she encountered. But the plan had not allowed for a robot city on the same planet. More intelligent than the Kin, SilverSides soon became their leader in the struggle against the robots. She launched a raid that crippled the city’s main planning computer, and, recognizing Derec as the leader of the robots, attacked him.

Only Derec’s invoking the First Law of Robotics saved him. But SilverSides was left with a dilemma. Were not the Kin human? How could they and Derec be human, and protected by the First Law? SilverSides took on the form of a human and the name Adam, but before this problem could be resolved there was another distress call-from Ariel. Joined now by Wolruf, Derec, Mandelbrot, and Adam went to her aid.

In Derec’s absence, Ariel had gotten a call from yet another robot city. This one was also under attack by aliens, but aliens of a kind vastly different from the Kin.

Ariel found this robot city almost completely enclosed by a dome. This planet’s inhabitants, the bird-like Ceremyons, were as advanced, compared to humans, as the Kin were primitive. Rather than attacking the city directly, they were sealing it under a dome where it could do no harm. The robots, following their programmed impulse to build and to prepare the planet for human habitation, were arranging to rebuild the city at a different location.

As soon as Ariel arrived, she summoned Derec through his internal connection with all the robot cities. But by the time he reached this planet, she had reached a tentative compromise-the Ceremyons, living almost all their lives in the air, would allow the robots to use some of the ground for farming, and they would allow one small enclosed city for the export of the food. Derec, with the help of the supervisor robots, reprogrammed the city.

Adam, still having no clear definition of what a human being is, imprinted on the Ceremyons, but they, needing no protection and having no need of his services, sent him back to Derec. Not yet certain to whom he owed Second Law obedience, he voluntarily set up his own agricultural experiment. In the course of this isolated work, he encountered a great silvery egg-an egg that he recognized as another being like himself, but not yet imprinted. Rushing back to the robot city, he brought Ariel to the egg in time for the new robot to imprint on her. Thus was Eve born.

Eve also went through the trauma of imprinting on the Ceremyons, but she encountered one who convinced her that he and he alone was human. Only his increasingly obvious insanity freed her from that dangerous illusion.

The agricultural reprogramming finished, Derec and Ariel and Wolruf decided to remove Adam and Eve from all possibly harmful influences-they would all go back to Robot City.

They returned to a Robot City in shambles. An unknown influence had seized control of the city’ s central computer, and tiny artificial humans-a few inches tall-were tucked away in many of the buildings. The robots had turned from maintaining the city to wild experimentation that reminded Derec and Ariel of the days of Lucius.

The obvious culprit was Dr. Avery. Although the experiments were of the sort that he had abhorred, he was the only one Derec knew who could seize control of the city. But while Avery did turn up in the city, he was so angry over the changes that he could not have been responsible. He was also no longer responsible for his own actions; he was now completely mad, convinced that he was turning into a robot.

Ariel took charge of the homunculi, and of Dr. Avery. She was more successful with Avery than with the tiny people, effecting the beginnings of a cure. Derec and Mandelbrot, meanwhile, tracked down the invading presence, an intelligence that called itself The Watchful Eye. This intelligence, it appeared, was guiding all the bizarre experiments in the hope of discovering the nature of human beings-and whether it might be one.

With the city collapsing around them, all forces joined to corner The Watchful Eye in its hidden lair. Finding it disguised as an ordinary piece of furniture, they at last forced it to reveal and face its true nature: the third of Dr. Anastasi’s “learning machines. ”

Taking the name Lucius II, the new robot immediately entered an intense exchange of information with Adam and Eve. To the already unresolved question of what constitutes a human being, Lucius II added the possibility that these three robots may be humans.

These discussions took place in isolation from the humans and Wolruf. They were concerned with the issue of what to do with the packs of small, rodent-like animals that roamed the streets, a residue of some of Lucius II’s experiments. Although they were clearly not human, these creatures had been generated using human genetic code as a starting point. Were they, then, also human, or could they be treated as vermin? This problem is complicated by Ariel’s pregnancy, and the discovery that the fetus has been damaged by Derec’s chemfets.

None of the medical robots on Robot City would even consider an abortion, since they considered the fetus human, even though it lacked a complete nervous system and could not survive birth. Adam offered to perform the operation in return for transportation back to the planet of the Ceremyons. The three learning machines hoped to consult with the Ceremyons on the question of humanity.

Robot City created a ship, which Dr. Avery named the Wild Goose Chase, from its own material. Surviving an accident that threatened all their lives, and Wolruf’s definition as human, they reached the planet of the Ceremyons to discover that their elaborate plans had been canceled. Someone-a woman, and apparently a brilliant roboticist-had come and helped the Ceremyons reprogram the entire city. Derec and Dr. Avery tried to adapt the city to serve the Ceremyons, but at last the natives could find only one useful purpose for it. As the humans, Wolruf, and the robots left for the planet of the Kin, they saw the robot city slowly melting into itself, and taking on its new form as a vast metallic sculpture.