The Enterprise is in orbit around a planet of molten lava with a poisonous atmosphere, which makes the life reading from the surface most perplexing. Even more confusing is the appearance of Abraham Lincoln on the ship's scanners.
Beaming the president on board, Kirk is amazed that Lincoln appears to be exactly who he claims to be. Spock and Kirk agree to accompany Lincoln to an area on the planet's surface that has transformed into an Earth-like environment; there they encounter a rock creature that seeks to understand about good and evil, in particular, which is stronger. To that end, it pits Kirk, Spock, Lincoln, and famous Vulcan Surak in a fight to the death against four historical villains.
This episode reflects moments in America's history that have demanded sacrifice, wisdom and bravery from its leaders in order to try and bring about equality and peace. Lincoln is obvious -- he united America -- but at what cost? Surak, on the other hand represents those who have taken a more peaceful stance against prejudice and violence and who have paid the ultimate price for their beliefs, in particular civil rights leader Martin Luther King, a recent tragic figure at the time this show was made.
That said, what I REALLY liked about this episode was the cool rock monster -- a really bizarre creature with a mean pair of claws and multiple eyes that flash in time with its voice -- and the fact that Lincoln goes mano a mano against Genghis Khan (would have loved it even more if Kirk had shouted 'Khaaaaaan!' at some point).
6.5/10, rounded up to 7 for IMDb.