the story: Scotty is retrieved from a transporter, and meets the next generation.
similar to: "Sarek," "Unification" (Next Generation),"Blood Oath" (Deep Space Nine), Star Trek Generations, "Flashback" (Voyager)
my thoughts: Because of the old age makeup they put him in, you wouldn't really know that DeForest Kelley (McCoy) appeared in Next Generation's premiere, "Encounter at Farpoint." At any rate, it's a far, far less famous appearance than Scotty's in "Relics." I mean, outside of Sarek and Spock's appearances, Scotty popping in out of nowhere ranks as one of the most charming memories from the franchise, likely because his appearance was keenly shaped around its nostalgic appeal whereas his Vulcan predecessors (and McCoy) were presented as contemporaries.
Instead, Scotty is literally ripped out of time and, like Rip Van Winkle, finds himself in totally unfamiliar surroundings. His holodeck visit is the first time the bridge of the original Enterprise is seen since, well, the original series. The whole thing is like a preview of Star Trek Generations (including a crack about where the upstarts were when the originals were out saving the galaxy), in which, you may remember, Scotty also appears (ironically, like Kirk, that trip aboard the "bloody" Enterprise-B is probably one of his last experiences in his own time).
Later crossover episodes couldn't really duplicate the magic. The original Klingon actors in Deep Space Nine's "Blood Oath" probably comes closest, but that's for the more hardcore fans (there really are levels of Star Trek geekhood), while Sulu's in Voyager's "Flashback" specifically revisits a familiar experience (Star Trek VI: The Undiscovered Country). When a Next Generation character appears in another series (which actually happened pretty frequently), it was not really considered too special (famously, fans considered this phenomenon as ruining Enterprise's final episode, "These Are the Voyages..."), although Voyager's Doctor got in a few good ones (Star Trek: First Contact and Deep Space Nine's "Doctor Bashir, I Presume?").
Getting back to "Relics" itself, more than all those other examples, it's actually kind of a commentary on the aging nature of the original series and its cast, arguably more so than what was found in the films (Star Trek: The Motion Picture and Star Trek II: The Wrath of Khan, both of which were a decade old by the time this episode aired). The fact that it aired a year after Undiscovered Country was also a sign that the old days really were gone (but only a few years before they turned up one last time in Generations).
And to link "Relics" with the new cinematic era, it's exactly like Old Spock showing up in 2009's Star Trek. That's all the context you need.
Apparently the science of it was also pretty cutting edge; the Dyson Sphere that fascinates both Scotty and Geordi (this is an excellent episode for him, too, finally putting to bed the challenges of presenting a second, and unique, genius chief engineer) could very well have carried an episode on its own.
It's that one scene, however, when Scotty visits the holodeck, and revisits the bridge of his old ship, with Picard joining him, that defines "Relics" as a classic. It's one of those moments that speaks for itself. And finally, finally, Scotty has his worthy showcase.
criteria analysis: franchise - series - character - essential (all criteria met)
James Doohan (Scotty)