Star Trek franchise's most enduring story mechanics, with the Mirror universe appearing throughout various eras. Star Trek: Discovery incorporates the Mirror universe in its very first season, and the Terran Emperor, Philippa Georgiou, follows the Discovery crew back to the Prime world. Star Trek: Discovery season 2 finds Georgiou acclimatizing to her new surroundings, and in the final episode, she opts to travel into the far future alongside Michael Burnham, the doppelganger of her adopted daughter.
Unfortunately for Georgiou, traversing universes and jumping through time is not healthy for the body, especially since the 32nd century Prime world has drifted apart from her Mirror reality. With Culber and Kovich at a loss, the Discovery's super-intelligent computer system finds only one possible solution - the Guardian of Forever. With mastery of time and space, the God-like Guardian of Forever (Carl to his friends) can return Georgiou to a timeline that won't cause her body to tear itself apart, but this solution is only decided after a test of the Emperor's mettle. Carl sends Georgiou back to the Mirror universe to see how much she's really changed.
Suddenly back on the throne of the Terran Empire, Georgiou must resume her brutal, ruthless persona from Star Trek: Discovery season 1, especially with Lorca's coup brewing in the background. The conspirators already thought Georgiou was getting soft before her trip to the Prime universe; now she must maintain the pretense of a dictator without arousing suspicion or showing weakness. Naturally, this proves impossible. Having reconnected with Prime Burnham, Georgiou will not kill her daughter in the Mirror universe. And after coming to respect Saru, the Emperor is more liberally-minded towards Terra's Kelpien slaves. Despite desperately trying to be the same, bloodthirsty Georgiou from before, Philippa simply can't hide the kinder traits she picked up in the Prime universe, and this concept riffs heavily on Star Trek's very first Mirror episode, "Mirror, Mirror."
The Original Series episode sees Kirk, Bones, Uhura and Scotty switched with their Mirror counterparts due to transporter malfunction (what are the chances?) and Kirk suddenly finds himself leading the ISS Enterprise. His Mirror crewmates expect Kirk to be unforgiving, violent and sadistic, and the Prime Enterprise captain does his best to act the part. Like Georgiou, however, Kirk simply can't ignore his principles, and this arouses suspicion. He delays attacking a peaceful planet, puts a stop to Chekov's torture and refuses to let bearded Spock be killed. Akin to Georgiou in Star Trek: Discovery season 3, Kirk is forced to play the part of someone more aggressive than himself to assimilate into the Mirror world, but can't avoid being found out when his morals risk being compromised. The end sequences of both "Mirror, Mirror" and "Terra Firma Pt. II" also employ the same note of optimism. As Georgiou departs back through Carl's gate, she tries to make the Terrans less Terran-y. Kirk makes exactly the same gambit shortly before transporting back home.
Modern Star Trek is often accused of straying too far from the original format and tone, but the Mirror universe plot of Star Trek: Discovery season 3 provides a modern reflection of an Original Series classic, with the spin that Georgiou is impersonating her former self, rather than a parallel counterpart, as Kirk was. This fits with Star Trek: Discovery's more character-based approach, but proves that, despite rocketing into the franchise's far future, season 3 is more "familiar" to Star Trek fans than ever, going where no Discovery season has gone before.
With Star Trek: Discovery giving its "Mirror, Mirror" cover version, not to mention bringing back the Guardian of Forever, season 3 is using Kirk's era and legacy more effectively than when the show ran a mere 10 years behind The Original Series. The comparison between the episodes falls somewhere between loving homage and keen inspiration. Georgiou and Kirk had very little in common when Star Trek: Discovery season 3 began, but after her return to the Mirror universe and an encounter with Forever, the Terran Emperor could perhaps relate to Shatner's captain more than anyone else in the current era.