Mirror, Mirror
Parallel worlds are a long-held sci-fi trope and while Star Trek can’t claim to have invented the notion, it certainly offered perhaps what is still the most iconic example of the alternate reality episode even after all these years, with countless future genre series paying homage to TOS‘ “Mirror, Mirror.”
It sees Kirk, Bones, Scotty and Uhura accidentally sent to another universe after being caught in an ion storm. This mirror universe has the evil Empire in place of the kindly Federation, with the Enterprise crew’s own doppelgangers being much different. Most famously, we have the Mirror Spock, complete with goatee. In fact, we practically have this episode to thank for the goatee now being the universal sign of a bad guy.
Though such a famous part of Trek lore, the Mirror universe surprisingly hasn’t been visited all that much beyond this episode. It has appeared in a handful of DS9 and Enterprise episodes, though, as well as playing a key role in the contemporary Discovery in a way we won’t spoil here.