After keeping us in the dark for almost 10 years, Stranger Things has finally revealed what the Upside Down actually is – and as Dustin Henderson says in season 5, all our theories about the dark realm have been "dead wrong" so far.
Well, unless you've seen the stage show, Stranger Things: The First Shadow, that is...
Warning! The rest of this article contains major spoilers for Stranger Things season 5.
At the start of Stranger Things season 5 Vol. 2, Nancy, Jonathan, Steve, and Dustin ignore Hopper's orders to wait at the church and, instead, take it upon themselves to investigate the great wall they crashed into inside the Upside Down. Brainbox Dustin suggests that the wall is likely a dark magic shield put there by Vecna; an extra precaution for his base, much like the Empire's energy shield around the Death Star II in Star Wars: Return of the Jedi. If he's right, he notes, they might able to find the shield's generator, shut it down, infiltrate Vecna's true lair and save Holly.
Their mission takes them to Hawkins Lab, in which Dustin stumbles across Dr. Martin Brenner's old research. With the files, the youngster learns that the Upside Down is a wormhole (or interdimensional bridge) between the real world and a mysterious new one they dub 'The Abyss'. Said dimension is presumably where the demogorgons, demobats, and the Mind Flayer hail from, as well as where Eleven banished Henry Creel/One during their psychokinetic battle inside the lab's Rainbow Room back in 1979.
It was there that he was trapped for four years until Eleven made remote contact with a demogorgon – and the Upside Down tunnel was formed. Since then, Henry/One/Vecna has been using the Upside Down as a entryway to the real world, making gates in an attempt to destabilize it and ultimately combine the Abyss with Hawkins. In short, the big bad wants to turn Earth into his own monster-laden hellscape.
"Netflix wanted us to explain the mythology to them, because we were very adamant early on, 'We don't want to explain it in the show. We like that there's mystery, and that there's a lot that you don't understand by the end of the season,'" Matt Duffer recently explained to Variety. "They said, 'That's fine, but we would like to know.' I think it was actually a really good exercise – we spent quite a bit of time with our writers figuring out exactly what the Upside Down was.
"We wrote a 20-page mythology document. It wasn't called the Abyss at that point; it was called Dimension X, which is a Ninja Turtle reference. We've been holding those cards back for so long; it was a real relief to actually be able to show our hands here."
Now, if you've been lucky enough to see The First Shadow already, you'll have heard of 'Dimension X' before. The play, which fleshes out Henry's backstory, opens with a prologue set in 1943, in which a warship called the USS Eldridge gets sucked into Dimension X. Said ship was commandeered by Captain Brenner, Martin's father and the sole survivor of the otherworldly disaster; his crew members having been picked off by malevolent smoky entities.
While it made no reference to the fact that Joyce and Hopper went to school with Henry, a major plot point in the play and something that was hinted at in Vol. 1, Vol. 2 featured another moment that stage show attendees will have a little more context for. During their exploration of Vecna's Mind Lair, Max and Holly wander into his worst memory and see a young, Boy Scout-uniformed Henry come across a bloodsoaked Soviet spy inside a mine shaft.
Terrified, the spy shoots Henry in the hand, prompting the boy to fatally bludgeon the older man with a rock and pinch the metallic briefcase he'd stolen from the US military. In the play, it's explained that a dazed Henry, who had been missing for 12 days, was found with a wound on his hand just outside a cave system in Nevada. It also spells out that the Russian was found dead inside the caves a little while later. While the Netflix show didn't divulge what's in the briefcase, those who've seen the play will know that it contains rift-opening tips surrounding Dimension X and other top-secret technology.
In the play, it's established that the contents of the briefcase transported Henry to Dimension X, leading to his first-ever encounter with the brain-scrambling Mind Flayer. A little while later, Dr. Brenner connected the dots between Henry and the Mind Flayer after finding the former's spyglass, the one Holly has been using in the TV show, in the caves.
Stranger Things season 5 Vol. 1 and Vol. 2 are streaming now. Don't miss the finale with our guide to the Stranger Things season 5 release schedule. If you need help figuring out exactly what's gone down so far, check out our breakdown of the Stranger Things season 5 Vol. 2 ending explained.
