Star Trek: Discovery's final season has hit the halfway point with the fifth episode "Mirrors", which featured several major revelations, notably about who one of the key villains and what species of aliens he belongs to. That wasn't the only major thing we saw in this latest episode, however.
While chasing after villains Moll and L'ak, Michael Burnham and Cleveland Brooker find themselves in a wormhole where they find the ISS Enterprise, the Mirror Universe version of the U.S.S. Enterprise, captained of course by Christopher Pike. The Mirror Universe's ISS Enterprise, on the other hand isn't captained by Pike, but by James Kirk. At least it was the last time we as an audience saw it.
While the ISS Enterprise technically became part of canon after the events of Star Trek: Discovery's first two seasons, with the time jump, the ISS Enterprise has been lost to time, at least until Burnham and Cleveland found it. First appearing in the season two episode, "Mirror, Mirror", the ISS Enterprise was home to the Mirror Universe versions of the USS Enterprise, with the close of the episode closing the story on the Enterprise until the 32nd century.
So what happened to the ship? A group of refugees apparently used the ship at some point after the events of "Mirror, Mirror" but had to abandon the vessel in the same interdimensional space wormhole that Cleveland and Burnham found it in.
It's a cool Easter egg but what's even cooler is that the set used for the ISS Enterprise is in fact the same set used on Star Trek: Strange New Worlds set. Since Discovery served as the starting point for the Anson Mount-centered spinoff, it was a nice treat to see Discovery call back to the ship (and subsequently the show) that they helped launch.