After completing a routine charting mission on an apparently lifeless planet with a molten surface and a poisonous atmosphere the Enterprise is about to leave when a strangely familiar figure appears on the main viewscreen, claiming to be Abraham Lincoln. Kirk humours him and orders him to be beamed aboard with full presidential honours.

Spock advises Kirk that the transporter appears to have locked on to a kind of living rock but, when the figure materialises on board, McCoy's instruments confirm that his readings are human. 'Lincoln' is given a tour of the Enterprise, exclaiming amazement and congratulating Kirk on their achievements. He invites Kirk and Spock to beam down to the planet with him, which they do, despite Scotty and McCoy's objections. Although the ship's sensors showed the surface to be molten and the atmosphere as unbreathable, they materialise in an area that appears to be quite Earth-like but their phasers have somehow disappeared during transit.

Another humanoid figure approaches the three of them, which appears to be the historically famous Surak of Vulcan. One of the surrounding rocks shows itself to be alive and announces itself to them as an Excalbian called Yarnek. It explains to them that the Excalbians want to examine the concepts of good and evil, which they do not understand, telling Kirk and Spock that they intend to carry out an experiment to see which is the stronger. Scotty calls Kirk to advise him that all power on the Enterprise is down and that they cannot be retrieved for the present.

The scenario of the alien 'experiment' is to pit two groups against each other, one 'good' and one 'evil'. Kirk is to lead the 'good' group, which consists of himself, Spock, Abraham Lincoln and Surak. Their opponents are to be Genghis Khan, Kahless and Zora, led by the evil Colonel Green. Kirk's team manages to survive the first attack following which the other group moves away to establish a base. Surak believes that a peaceful solution can be found and goes to the other camp to negotiate. Kirk and the others hear his screams and make a rescue attempt but he is already dead. Abraham Lincoln also dies during this assault.

Kirk and Spock eventually manage to triumph and Yarnek reappears to examine the results of the Excalbian experiment. He still cannot understand the essential difference between good and evil although Kirk tries to explain that Colonel Green was fighting to achieve power while his own intention was purely to save the lives of his crew. On being asked by Kirk what right the Excalbians had to test them Yarnek replies that it is the same right that Kirk's people have to explore the galaxy. The Excalbians restore power to the Enterprise and allow Kirk and Spock to depart.