After witnessing a spaceship and crew of one commit suicide by diving into the star that Deneva orbits. Capt. James Tiberius Kirk discovers the Pilot was suffering from an infection which is paralysing the people of Deneva and surrounding planets.
The crew visits the planet and discovers many have died from the infection including Kirks brother. When Spock is attacked by the virus, Kirk has to deal with the consequences of allowing him to live and infect many.