What was the first Gunsmoke episode that Ken Curtis was in?

Ken Curtis made his first appearance in "Gunsmoke" in the episode titled "Doc's Revenge," which is the 13th episode of Season 8. He portrayed the character Festus Haggen, who would later become a prominent and beloved character in the series. Curtis's introduction added a new dynamic to the show, and Festus became a staple alongside Marshal Matt Dillon.