The Enterprise is assigned to investigate a space/time rift that has swallowed up a spacecraft. Spock believes there is a connection with the Taygetians, an aquatic race who spend all their time singing: The Federation has designated them animals and allowed them to be hunted since they shed a crystal tear when they die. To try and communicate with them, Kirk forcibly recruits musician Guy Maslin, even though he suffers from a disease that could be fatal if he is overworked or stressed. En route to the system, Maslin and Uhura fall in love and she considers leaving Starfleet to be with him.