The Enterprise arrives at Deneva – the home of Captain Kirk’s brother, Sam and his family – and discovers that the entire planet has been infested with large, Amoeba-like aliens that have attacked and killed much of the human population. One of these aliens attaches itself to Spock, who volunteers to become a subject in Dr. McCoy’s medical tests. Can McCoy and Kirk find a cure in time to save Spock and the remainder of the Denevan population?