Knowing that in real history the gunfight was fatal to most of the Cowboys, the "Enterprise" crew make several attempts to alter their fates, but their efforts are unsuccessful, leading them to believe that history cannot be changed. However, when one of the townspeople, Sylvia, gets close to Chekov, Morgan Earp interferes and kills Chekov. Spock remarks that the real Billy Claiborne had survived, suggesting that the day's events could be changed in other ways. To that end, Spock creates an improvised tranquilizer gas grenade to subdue the Earps before the shootout, and is surprised when the gas fails to work.