After two years in prison for selling drugs, Tom More, a Louisiana psychiatrist and lapsed Catholic, comes home to his virtually defunct practice and marriage. People, he notices, seem different. Many faculties are dulled but some are enhanced, particularly memory and sexual appetite.
Teaming up with his cousin Lucy, an epidemiologist who also has an independent mind, they discover that the authorities have been running secret trials. Through the addition of sodium ions to the water supply, the active population is gradually being made more chimpanzee-like, while the inactive, the old and the sick, are being euthanased. To Tom's particular disgust, the leaders of the trials are found to engage in sexual abuse of children, for which he takes his revenge by forcing them to drink high concentrations of sodium and so regress to apes.
A parallel plot involves a Catholic priest, Father Smith, who like Tom has hit rock bottom and almost totally failed in his calling. Together, with difficulty, the two men rediscover the hope hidden in their shaky faith.