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.
The big meta quote
While trying to gee Stamets up, Lorca declares “You chose to go where no one has gone before.” I don't have to explain where that one comes from, do I?
So, they're in the mirror universe now, right? Stamets talks about seeing “infinite permutations”. At a guess he's talking about realities and the Discovery is now in a different universe. That would explain Saru's inability to determine where they are – and possibly why we've never encountered a ship with this technology before...
RIP in peace, U.S.S. Gagarin
The episode opens with a fantastic battle sequence featuring a new Federation starship, the U.S.S. Gagarin. The Shepard class starship (the same model as the U.S.S. Kerala from episode 2 Battle at the Binary Stars) is, of course, named after Yuri Gagarin - the Russian cosmonaut who was the first human to enter outer space.
This isn’t the only Trek starship to be named in honour of the great man. The U.S.S. Yuri Gagarin is listed in the Next Generation episode The Measure of a Man as a Federation starship, patrolling the Neutral Zone. There’s also the VK Yuri Gagarin, an early human DY-732-class starship that served in the early 22nd century.
Gagarin also has at least one planet (and presumably several more) named after him in Trek lore. Gagarin IV is named-checked in the Next Generation’s Unnatural Selection as the home of the Darwin Genetic Research Station.
There’s a lot of talk about the Klingons’ cloaking technology in this episode. At this point in galactic history, the Romulans are well-versed in cloaking tech, but it’s still something of a novelty to Klingons. Except, this episode establishes a rather interesting wrinkle in the continuity. Now, Klingon cloaking tech is derived from T’Kuvma’s Sarchophagus ship - ancient technology that seems to have been lost to the modern-day Klingons. By the time of the Kirk films, Birds of Prey with cloaking tech will be commonplace. It seems we have Kol to thank for that.
"The needs of the many…"
In The Wrath of Khan, Spock espouses Vulcan logic, stating “Logic clearly dictates that the needs of the many outweigh the needs of the few”. By the end of the film it will have taken on a tragic meaning, as he sacrifices his life for the Enterprise crew. In the next film, The Search for Spock, Kirk flips this statement on its head, risking everything to save the life of one man - his newly-resurrected Vulcan pal.
This episode echoes and - technically - foreshadows those scenes. Burnham says, “The needs of the many...” only for Tyler to interject with “...Are worth fighting for. Are worth dying for. But so are the needs of the few.” It’s a nice moment, but you have to wonder if it’s subtly hinting that one or both of these people may end up sacrificing their lives for the greater good.