Star Trek: Discovery may have sent their crew 900 years into the future, but they remain closely tied to Star Trek: The Original Series, as shown in Thursday's episode "Terra Firma, Part 2," which is also Michelle Yeoh's farewell episode.
This season has seen a major emphasis on the Mirror Universe, aka the alternate dimension full of "evil" doppelgangers of our Prime Universe — most memorably, the one where Spock (Leonard Nimoy) has a goatee. In Discovery, Philippa Georgiou (Michelle Yeoh) has died in the Prime Universe, but her Mirror counterpart has taken her place, and has had to learn how to exist in a reality where absolute ruthlessness is not the norm.
As hinted at in the original "Mirror, Mirror" episode, denizens of the Mirror Universe are not inherently evil, but society has evolved in a much less civilized way. That concept has been emphasized by Mirror Georgiou learning to let down her guard and grow ever closer to Prime's Michael Burnham (Sonequa Martin-Green). However, in the most recent episodes, Georgiou has had to seek help to cure her molecular instability caused by being so far removed from her dimension of origin. The help comes in the form of a mysterious figure calling itself Carl (Paul Guilfoyle), who has deemed that the time has come for Georgiou to return to her universe.
Why does Carl get to make that call? Well, the fact that he's dressed like he's from the 1930s should have been a clue, but he reveals himself to be the Guardian of Forever, the all-knowing voice of a sentient portal to anywhere in time and space.
This is a callback to perhaps the best TOS episode, "The City on the Edge of Forever," where Captain James T. Kirk and Spock travel back to pre-World War II Earth, and Kirk falls in love with Edith Keeler (Joan Collins), but has to let her die in order to preserve history.
This time, Georgiou is sent back to the Mirror Universe, but she's not the same Emperor of the Terran Empire she used to be. Her efforts to change her ways there result in her death, but it turns out that experience was only a test. The Guardian of Forever deemed that she passed, and thus she is allowed to go somewhere else, where she will live on.
So Georgiou bids an emotonal farewell to Burnham, and Yeoh does the same to the cast and crew of Star Trek: Discovery.
New episodes of Star Trek: Discovery premiere Thursdays on CBS All Access.
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Andy Hunsaker has a head full of sitcom gags and nerd-genre lore, and can be followed @AndyHunsaker if you're into that sort of thing.