"Isik for your thoughts?" Burnham says this Vulcan proverb to the traumatised Tyler in this episode, who responds by asking what an isik is. Turns out that it's a type of Vlugtan currency, first mentioned in Deep Space Nine's Rivals.
Who is the "faceless emperor"?
While giving Lorca and the crew the full mirror universe info-dump, Burnham mentions that the Terran Empire is ruled over by a mysterious emperor. We know from previous stories that there have been several different holders of this title. In the 22nd century, the Enterprise’s Hoshi Sato seized power when she commandeered the USS Defiant (more on that in a moment).
The following century, the mirror universe Kirk tried to take charge, but was assassinated by mirror Spock, who went on to become emperor. Following his sojourn to the prime universe, he attempts to make the Empire a kinda, less fascistic sorta place. Unfortunately, it doesn’t go well for Earth in the long-term. Spock's reforms accidentally open a door for the Klingon-Cardassian Alliance to enslave both humanity and the Vulcans. Whoops…
So who is the emperor/empress in Discovery’s time? If the show wants to go full on fan-service then they could make it one of Sato’s relatives - the novels suggest that there are three generations of Empress Satos. However, given that we know Michelle Yeoh is going to be an ongoing part of this show - despite her character being extremely dead - I'd lay an isik or two on the Empress being none other than Georgiou.
To get back to the prime universe, the crew realise that they will need to examine another ship that has made the transition between realities: The Constellation-class U.S.S. Defiant, NCC-1764. And that’s a ship with a lot of history...
The Defiant is first seen in the Original Series episode The Tholian Web. The Enterprise discovers the ship with its entire crew dead and is unable to retrieve it due to interference from the Tholians. What happened to the ship next was the subject of fan speculation for decades until Enterprise revisited the craft in the In a Mirror, Darkly episodes. There, it is established that the Defiant was pulled both back in time and into the mirror universe, where it is captured by Captain Archer of the I.S.S. Enterprise. He doesn't have the ship for long, however. As previously mentioned, Sato kills Archer and uses the ship in her bid to take control of the Empire.
In the Next Generation timeline there is, of course, another U.S.S. Defiant - the "tough little ship" stationed at Deep Space Nine and used to tackle the Borg in First Contact. They’re related by name only.
In the future, all chief engineers will be Scottish
Lorca gets a rare loveable moment this episode when, pretending to be the I.S.S. Discovery's chief engineer, he adopts a Scottish accent not a million light years away from James Doohan's beloved Montgomery Scott from the Original Series.
Pain, no gain
The agonizer booth that Lorca finds himself in at the end of the episode first appears in Mirror, Mirror, and again in Enterprise’s In a Mirror, Darkly, where it’s revealed to have been created by Dr Phlox and Malcolm Reed.
It's been a tough few months for Captain Lorca – he may have been helping turn the tide of battle against the Klingons, but as we saw in his last encounter with Vice Admiral Cornwell, it's only a matter of time before his shadier aspects come to light. Not today, though. His plan to use Stamets and the DASH drive results in a decisive victory. His reward? Starfleet's Legion of Honor.
He's not the first regular Trek character to win it though. Both Dr McCoy in the Original Series' Court Martial and Data in The Next Generation's Measure of a Man are stated to have received the commendation. Lorca, for his part, tries to pass it off to Stamets.
The scene where Stamets performs 133 spore-jumps seems to consciously nod to Spock's iconic death scene in The Wrath of Khan. As Stamets' chamber is flooded with spores, fundamentally altering his DNA and nearly burning him out, Dr Culber watches on in horror, his hand pressed against the glass – just as Kirk and Spock did before them in Khan (and in Into Darkness's sloppy reprise of the scene). Happily, Stamets doesn't die.
One of the more classic pieces of Star Trek technology gets a big moment here. The universal translator has been in every iteration of the franchise, even showing up in Gene Roddenberry's Star Trek Is... pitch document for the very first series, which states: “We establish a ‘telecommunicator’ device early in the series, little more complicated than a small transistor radio carried in a pocket. A simple 'two-way scrambler', it appears to be converting all spoken language into English.”
The original Enterprise NX-01 has an experimental version of the technology, though still requires a human translator in the form of the ship's linguist and communications officer, Hoshi Sato. In Discovery, the technology seems to have become standard Starfleet issue.
A historic moment
Not specifically an Easter egg, but Into the Forest I Go is notable for featuring the first romantic kiss between two men in Star Trek history. Deep Space 9 previously featured the first lesbian kiss in the franchise, in the episode The Emperor's New Cloak.
Admiral Terral tells Lorca that Cornwell is heading back to this location to receive medical treatment. While it's not been mentioned in the main Trek canon before, it does show up in Michael Jan Friedman's novel, Planet X – an unlikely crossover between The Next Generation and - wait for it - the X-Men. No, seriously. Luckily it's the comic version of Marvel's mutants, so Picard doesn't find Professor X strangely familiar.