Baley is unexpectedly contacted by Daneel to help resolve an authorship dispute between two Spacer scientists. Being Spacers, neither scientist is willing to allow himself to be interrogated by an Earthman, but they are willing to allow Baley to interrogate their personal robots. The two robots are the same model, and their stories are mirror images of each other: each one insists that his own master came up with a key scientific insight, and that the other scientist is falsely trying to lay claim to it. Clearly, one of the robots is telling the truth, while the other has been ordered by its master to lie. Baley must use the Three Laws of Robotics and his own knowledge of robot nature to determine which is which. But the answer really comes down to his knowledge of human nature.