On an asteroid far from earth, work on developing a hyperdrive which would free humanity from the confines of the solar system has come to a halt. A robot with a weakened First Law—prepared at government insistence—has escaped and hidden itself in a shipment of outwardly identical unmodified robots. Since it might conceivably be dangerous, it cannot be allowed to escape, so Susan Calvin and Peter Bogert are sent out to figure out which is the modified robot.

This is a terrific story, among the best of the Susan Calvin tales. (The fact that it was written in the late 1940’s helps, after some of the stories it precedes in I, Robot.) Calvin here is particularly interesting, because of her ability to handle and understand robots and because of her appreciation of what the Three Laws really mean. The tension is very real, too, as scheme after scheme meant to figure out which robot is which fails and the missing robot gets more and more dangerous.