"Through the Looking Glass"
Star Trek: Deep Space Nine
Remember "Mirror, Mirror," the episode from the original series where Kirk had a transporter accident and ended up in an alternate reality where Spock, however improbably, proves to be even more awesome with the simple addition of a goatee?
Well, Deep Space Nine returned to that alternate reality, repeatedly. First it was with "Crossover," in which Bashir and Kira discover how badly things turned out for humans after Mirror Spock took Kirk's advice and tried to course-correct the Terran Empire. Mirror Sisko, apparently, was something of a scoundrel. And then he died.
"Through the Looking Glass" is the second in this series of episodes (to be followed by "Shattered Mirror," "Resurrection," and "The Emperor's New Cloak"), but as it turns out, it's a great deal more than that.
You see, whereas Mirror Sisko is now unavailable, there's another Sisko over there. In the first episode of the series, we quickly learned Sisko's backstory, which involves the famous Battle of Wolf 359 (the Borg crisis as depicted in Next Generation's "The Best of Both Worlds") and how he loses his wife Jennifer, taking away from him his wife and from his son Jake, a mother. Except now, Sisko is brought to the Mirror Universe (that's what it's commonly called) and meets Mirror Jennifer.
It's a startling moment. Remember Next Generation's "Yesterday's Enterprise," in which we discover an alternate reality where Tasha Yar (a series regular from the first season) is very much still alive, which later leads to Sela, the Romulan daughter of Yar, and they're all played by the same actress (Denise Crosby)? It's the kind of continuity that seems completely impossible, but is one of the neater things Star Trek has managed to accomplish over the years (the best example will always be Leonard Nimoy popping up in two J.J. Abrams movies). Felecia M. Bell has far less pedigree than Denise Crosby or Leonard Nimoy, but she played Jennifer in the Deep Space Nine pilot ("Emissary") and then returns for an expanded performance in "Looking Glass" (and then encore in "Shattered Glass").
The result is amazing. Not so much for anything Bell herself does. For Sisko, it's a part of the whole rebuilding process he'd been experiencing (this is one of the many examples of everything the third season did right by Sisko).
It's a defining moment for the series and arguably an unheralded one for the whole franchise.