Kirk and Spock travel to Depression-era New York to repair a change in history that begins with a woman named Edith Keeler.
After a drug accident, Dr. McCoy escapes into the past through the Guardian of Forever, an ancient time portal, and changes history so the Enterprise vanishes. Kirk and Spock follow him to New York in the 1930s, where Kirk falls in love with a social worker, Edith Keeler.
Widely regarded as the best Star Trek episode ever made, it won a Hugo Award and was written by Harlan Ellison, though the aired version differed from his script. The ending turns on a terrible choice. Fans remember its famous final line from Spock and Kirk's heartbreak.