'Star Trek: Discovery': Everything You Need to Know About the Mirror Universe

Last week's episode of Star Trek: Discovery took the series into an alternate timeline. If all the talk of the mirror universe left you lost, we're here to help.

In this article, we'll get you up to speed on everything you need to know about the mirror universe. This should have you well prepared for tonight's new Star Trek: Discovery episode.

The earliest information we have about the mirror universe comes from Enterprise. The series' final season included a two-parter called "In a Mirror, Darkly." Unlike other mirror universe stories, these episodes didn't involve characters crossing over. Instead, it took place entirely in the mirror universe, following the mirror crew of the NX-01. These episodes are the most relevant to the plot of Star Trek: Discovery.

The first contact between humans and Vulcans happened differently in the mirror universe. "In a Mirror, Darkly" used footage from Star Trek: First Contact to show this. Instead of greeting the Vulcans peacefully, Zefram Cochrane shot the first one he saw. A mob of humans then stripped their ships for parts.

Humanity used the technology stolen from the Vulcans to find their way into space. Because they didn't have Vulcan aid, their engines released high levels of radiation. Nonetheless, the Terran Empire was soon born. Humanity rules the galaxy with an iron, xenophobic fist, enslaving other species. The other species of the galaxy band together to form a resistance. This rebel group has some success against the Terran Empire.

"In a Mirror, Darkly" reveals that the USS Defiant, a ship from the future of the prime timeline, crossed over. The ship was first seen in the Original Series episode "The Tholian Web." Jonathan Archer discovers this and mans an unsanctioned mission to claim the vessel. He succeeds, but Hoshi Sato betrays him and declares herself Empress.

By the time Star Trek's history catches up with the mirror universe again, the Empire seems to be at its height. Presumably, this is due to the powerful new technology discovered on Defiant.

The first Star Trek episode to feature the mirror universe was "Mirror, Mirror." This Original Series episode takes place about 100 years after "In a Mirror, Darkly". A transporter malfunction sends Captain Kirk and several crewmembers to the mirror universe. Their doppelgangers take their place in the prime timeline. Until Star Trek: Discovery, this was the earliest known contact between the universes.

The episode reveals that mirror Kirk is planning a coup of the current emperor. Prime Kirk meets mirror Spock and is able to use logic to convince him that there is another way. After they all deal with the transporter mishap and mirror Kirk, Spock begins preaching reform. The Empire becomes softer and gentler as Spock rises through the ranks.

This backfires. Star Trek: Deep Space Nine included several mirror universe episodes, beginning with "Crossover". The episode is set 100 years after "Mirror, Mirror". It reveals that a Cardassian-Klingon alliance overthrew the Terran Empire. Now they subjugate and enslave humans. The continuing mirror universe arc follows a new human resistance battling the alliance.

Based on these stories, the mirror universe isn't just an alternate history. It wasn't the Defiant's arrival that threw things off. That Cochrane was so hostile towards Vulcans suggests things changing much earlier. We also know from a comment by mirror Jonathan Archer that the Terran Empire existed on Earth before humans began space travel.

The change also isn't contained in the Terran Empire. The Cardassian-Klingon Alliance could have been the beginning of something like the Federation. Instead, it molded itself in the shape of the Terran Empire it had defeated.

In truth, the mirror universe seems to be less a mirror image of Star Trek history than a mirror image of its soul. The spirit of cooperation that led to the creation of the Federation is absent. Instead, there is an overriding spirit of self-interest and a lust for power.

There are some things about the mirror universe that it is best not to think too hard about. For one, doppelgangers can at times seen so bloodthirsty and ambitious you wonder how anything gets done.

Also, if history is so different, how did everyone's parents still pair up at the same time? How else do all these identical doppelgangers exist? This is particularly odd when you meet the mirror universe's Miles O'Brien. This Miles has never met his prime universe wife, Keiko. But they have children in the prime timeline so... do those people just not exist in the mirror universe? Wouldn't that wipe out an entire lineage? Is that a common thing? Everyone else seems to be there, right?

It's best not to get too bogged down in the details. Instead, join us in trying to figure out who the mysterious Terran Emperor is.

New Star Trek: Discovery episodes become available to stream Sundays at 8:30 p.m. ET on CBS All Access.