HT.030 The City on the Edge of Forever (TOS)

While investigating a strange rock donut portal, McCoy, OD’d on space molly, leaps back in time and butterfly effects the Federation’s very existence away.  To put things right, Kirk and Spock go back in time to stop McCoy from getting the Sports Almanac and ruining the future.  It’s the episode that that puts Roddenberry’s hopeful vision of the future front and center in stark contrast with 1930’s Depression-era New York.