"The City on the Edge of Forever" is in my opinion the best episode of Star Trek. The story of Kirk, Spock and McCoy's journey to 20th century Earth is fascinating from the very beginning and keeps the viewer's interest hooked throughout. What's also very good in this episode is how it is shown how man's actions can effect the course of time and what events occur. Edith Keeler's hopes and dreams for peace are noble, but what impact will they have on the world? This is one of the fascinating ideas this episode presents. There are many great episodes of Star Trek but you will not find another better than this.