When the planet Empyrea, a colony of genetically perfected human beings, demands that the Federation remove a science station which has been in place for nearly twenty years, the starship Enterprise is assigned to transport to the planet the Federation ambassador who negotiated with the Empyreans long ago - an ambassador who was once Dr. McCoy's closest friend, but is now a bitter rival.
On Empyrea, McCoy discovers Anna, a daughter he never knew he had. McCoy soon realizes that the isolationalist Empyreans must not learn her father is an off-worlder, and that her genes are less than "perfect". As relations with the Empyreans collapse around him, McCoy must find a way to save his newfound daughter from the harshest penalty her planet can impose!