What hasn't been said about "The City on the Edge of Forever"—considered by many as the all-time best episode of Trek? It's a true classic, with a poignant, tragic story and brilliant performances. The crew makes the great discovery of a time portal (the Guardian of Forever), but a demented McCoy—suffering from an inadvertent maddening-inducing medicinal-drug overdose—jumps into Earth of the 1930s and somehow radically alters history for the worse. Kirk and Spock follow McCoy through the portal to undo the damage.
In the past, Kirk and Spock are taken in by Edith Keeler (Joan Collins), whom Spock learns is destined to lead a pacifist movement delaying the United States' entry into WWII, thus allowing Germany to conquer the globe. The tragedy, as everyone knows, is that Kirk must let this warm, generous woman die in order to preserve history—even as he begins to fall in love with her.
Harlan Ellison's story, despite the controversy surrounding Roddenberry's alterations to it, makes a great hour of television with a social relevance and an emotional core that resonates. Shatner delivers one of his best performances, and Nimoy is terrific as the voice of reason while Kelley's manic raving is downright frightening. It's almost surprising that such a fully textured story fits within the confines of a single hour.