In the third season, Star Trek: Discovery connected to a past iteration of the franchise like no other series had before. In Season 3's "Unification III," the storytellers created a direct connection to Star Trek: The Next Generation's "Unification I & II," which featured Leonard Nimoy's Spock. However, the way Discovery used The New Generation footage could change the way viewers see all of Star Trek. "Unification III" is the first-ever "third part" of a two-part episode that began in another Star Trek iteration. In fact, the episode fixes the biggest mistake the Kelvin Timeline movies made, which is suggesting Spock's final mission to unite Vulcans and Romulans failed. This episode exists to show that it was a success, thereby bringing the final "big" enemy of The Original Series into the more peaceful galactic community.
Source：https://www.cbr.com/star-trek-discovery-tng-spock-footage-change-viewers-see/
