The Enterprise intercepts a stolen space yacht, whose thieves destroy it rather than face capture. But they are captured, and revealed as "counterculture" individuals looking for a mysterious planet they call Eden. Led by an insane genius, they manage to capture the Enterprise long enough to direct it to Eden, which is located in the Romulan Neutral Zone! And to make their escape, they create a lethal trap for the crew.

The Enterprise surveys the planet Excalbia. As its surface is the temperature of molten rock, no landing party is possible, so when the orbital survey concludes, Kirk orders the ship to leave. And then... Abraham Lincoln appears, and at the same time a region forms on the planet suitable for human life. Kirk and Spock beam down and discover Yarnek. It seems the Excalbians are struggling to understand the concepts of good and evil; the help themselves they have constructed an experiment: four examplars of good, including Kirk and Spock, will battle four exemplars of evil, notorious figures from history. And if good loses, the Enterprise will be destroyed...