10 Best Star Trek Episodes That Revisited Other Episodes

The mirror universe has been revisited tons of times throughout Trek since first showing up in the Original Series episode Mirror, Mirror. It popped up most commonly in Deep Space Nine and Discovery, but one of the most interesting appearances of this dark alternate dimension was in the Enterprise episode In A Mirror, Darkly.

This episode took place entirely inside of the mirror universe and showed us an authoritarian and evil version of the NX-01 crew seizing control of an Original Series style Constitution-class starship from the 23rd century called the Defiant (a different Defiant from the one in DS9). We came to learn that the Defiant was not only from their future, but also from an alternate reality: the prime universe. We were treated to beautiful recreations of Original Series sets and props, and even the long-awaited return of the Gorn.

You probably already know all of that if you watched the episode, but what you may not know is that this was actually not the first time we saw this ship. In the Original Series episode The Tholian Web, we saw how the Defiant ended up in a parallel reality. The Defiant got trapped inside a 'spatial interphase anomaly' that trapped them between universes and caused the crew to all go insane and kill each other. The Enterprise was called to investigate and, after a fight with the Tholians, the Defiant ended up getting pushed through the anomaly into an unknown dimension. It then came into the possession of the mirror universe Tholians.

Later, the Defiant was mentioned a few times in Discovery while the crew was trying to find a way to escape the mirror universe, but that ended up being unfeasible since the Defiant crew were driven insane from their crossing.