Ralph Messenger is a man who knows what he wants and generally gets it. As director of the prestigious Holt Belling Center for Cognitive Science at the University of Gloucester, he is much in demand as a pundit on developments in artificial intelligence and the study of human consciousness. Known to his colleagues as a womanizer, he has reached a tacit understanding with his American wife Carrie to refrain from philandering in his own backyard.

This resolution is already weakening when he meets and is attracted to Helen Reed, a recently widowed novelist who has taken up a post as writer in residence at Gloucester. Fascinated and challenged by a personality and a worldview radically at odds with her own, Helen is aroused by Ralph's bold advances but resists on moral principle. The standoff between them is shattered by a series of events and discoveries that dramatically confirm the truth of Ralph's dictum that "we can never know for certain what another person is thinking".