This is, in my opinion, THE best Star Trek TOS episode, and my favorite. This is one of four episodes (three if you don't count "The Naked Time") which feature time-travel, and the one which does it the best. Kirk and Spock go back in time via The Guardian Of Forever, after a very ill and very paranoid McCoy, who has somehow changed history, and attempt to fix it. Once in the 30s, Kirk falls in love with Edith Keeler, their "focal point in time".
This episode is one of the best balanced episodes, as a perfect mixture of suspense, drama, romance and humor. The story is unforgettable and well-written, the acting spot-on, and Shatner's last line ("Let's get the hell out of here") is my single favorite line of the Original Series. Love it.