Abraham Lincoln and Surak appear to help Kirk and Spock fight historical villains on a rocky planet.
The Enterprise is orbiting a volcanic planet when a being that resembles Abraham Lincoln appears. He and the Vulcan philosopher Surak are asked by a rock creature to fight alongside Kirk and Spock against a group of infamous villains from history, including Genghis Khan and Colonel Green, in a test of good and evil.
The episode is known for the appearance of Lincoln and for showing Spock's reverence for Surak, the founder of Vulcan logic. It raises questions about the nature of good and evil. Fans like the idea of historical figures at war, and the episode adds to Vulcan lore.