Star Trek: The Original Series dealt with everything from rogue scientists to godlike entities, so it's curious that the show's two highest-rated episodes both derive from the same core concept. The adventures of Captain Kirk's Enterprise crew are the stuff of legend almost 60 years later, as Star Trek: The Original Series is packed with stone-cold classics that still resonate strongly among the ever-expanding multimedia behemoth Star Trek has become.
Two of the most popular Star Trek: The Original Series episodes, and the two highest-ranked on IMDb, are "The City on the Edge of Forever" from season 1 and "Mirror, Mirror" from season 2. Both remain hugely enjoyable from a modern perspective, and both have carved out a legacy in subsequent projects, with the latter spawning an entire subsection of the franchise: Star Trek's Mirror Universe episodes. While both efforts share iconic status, they actually share far more, and these similarities highlight a deeper truth about Star Trek's capacity for storytelling.
Star Trek's "The City On The Edge Of Forever" & "Mirror, Mirror" Are Both "What If...?" Stories
The Same Basic Idea Applied Very Differently
At their core, Star Trek's "The City on the Edge of Forever" and "Mirror, Mirror" are both rooted in alternate histories - the notion that by altering a small part of the timeline, an entirely new reality can follow. "The City on the Edge of Forever" explores the past, as a wild Dr. McCoy inadvertently reverses a vital moment in Earth's development, creating a ripple effect that culminates with the disappearance of the Enterprise itself.
"The City on the Edge of Forever" and "Mirror, Mirror" both, in essence, represent a step away from the norm for Star Trek: The Original Series .
"Mirror, Mirror" debuted Star Trek's Mirror Universe, revealing a parallel universe in which the Federation took the form of an evil empire rather than a (mostly) benevolent coalition. From Spock's beard to Kirk's malice, almost every aspect of Star Trek's world was rewritten.
While the twin concepts of time travel and alternate realities are integral parts of Star Trek canon, they were never the bread and butter of The Original Series. The standard weekly format saw the Enterprise land on a planet, resolve whatever issue was occurring, then move onto the next adventure. "The City on the Edge of Forever" and "Mirror, Mirror" both, in essence, represent a step away from the norm for Star Trek: The Original Series. That deviation from the standard formula may partially explain why both episodes became so popular, although the excellent narratives and imaginative concepts certainly helped.
Why Star Trek's Format Is Perfect For What If...? Stories & Alternate Histories
It's Star Trek, Jim, But Not As We Know It
Since The Original Series, even more "what if...?" tales have been told across the various Star Trek TV shows and movies, from 2009's big screen reboot altering history to create the Kelvin timeline to Star Trek: Discovery taking an extended dip into the Mirror Universe. Following the standard set by The Original Series, these episodes have generally enjoyed positive reactions. Star Trek: Discovery season 1's two-part Mirror Universe adventure and Star Trek: The Next Generation's "Yesterday's Enterprise," for example, are both considered among the best Star Trek episodes.
Episodes like "A City on the Edge of Forever" and "Mirror, Mirror" break Star Trek 's one final frontier by blurring the lines of reality.
Perhaps the biggest reason Star Trek thrives when telling "what if...?" stories is because they break the only real rule the franchise has. Star Trek's space-faring premise allows for near-endless possibilities. Any planet or alien species the viewer can imagine might appear. Even so, Star Trek remains bound by both its own in-universe history and the real-world history that is supposed to have taken place beforehand. Episodes like "A City on the Edge of Forever" and "Mirror, Mirror" break this one final frontier by blurring the lines of reality, tossing history to the wind, and truly allowing anything to be possible within the Star Trek universe.
Another key reason behind the success of Star Trek's "what if..." episodes is the moral ambiguity invited by such stories. Star Trek is at its best when it tests the audience's ethical values, and stories that feature evil versions of good characters or alternate timelines are perfect for generating those conversations. The dilemmas over whether Edith Keeler should die or whether one twist of fate could turn Captain Kirk evil are among the most tantalizing questions in Star Trek history.
