Stranger Things Season 5 Unveils the Upside Down’s True Origin

For nearly a decade, fans have been left guessing about the true nature of the Upside Down. Now, in the latest season, the mystery is finally unraveled—and as Dustin Henderson bluntly puts it, every theory so far has missed the mark. Unless, of course, you caught the stage production, Stranger Things: The First Shadow, which dropped some major hints last year.

Breaking Down the Upside Down

Season 5 Vol. 2 kicks off with Nancy, Jonathan, Steve, and Dustin defying Hopper ’s orders to stay put at the church. Instead, they set out to investigate the massive wall they crashed into inside the Upside Down. Dustin, always the brains of the group, speculates that the wall is a magical barrier created by Vecna —similar to the Empire’s shield around the Death Star II in Return of the Jedi. If he’s right, they might be able to find the source, disable it, and sneak into Vecna’s lair to rescue Holly.

Their search leads them to Hawkins Lab, where Dustin uncovers Dr. Martin Brenner’s old research. Through these files, he discovers that the Upside Down is actually a wormhole—a bridge between our world and a mysterious new dimension they call ‘The Abyss.’ This is likely the origin of the demogorgons, demobats, and the Mind Flayer, as well as the place where Eleven banished Henry Creel (One) during their psychic showdown in the Rainbow Room back in 1979.

Vecna’s Plan and the Abyss Connection

Henry was trapped in this realm for four years until Eleven made contact with a demogorgon, which created the tunnel to the Upside Down. Since then, Vecna has used the Upside Down as a gateway to our world, opening portals in an attempt to merge the Abyss with Hawkins and turn Earth into a nightmare filled with monsters.

"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."

Stage Show Secrets and Deeper Lore

If you managed to see The First Shadow, you might already be familiar with ‘Dimension X.’ The play, which dives into Henry’s backstory, opens in 1943 with the USS Eldridge being pulled into Dimension X. The ship, led by Captain Brenner (Martin’s father), is left with only one survivor after the crew is attacked by shadowy entities.

While the play doesn’t mention that Joyce and Hopper went to school with Henry—a key plot point in the show—it does provide extra context for certain moments. For example, in Vol. 2, Max and Holly explore Vecna’s Mind Lair and witness a young Henry encountering a bloodied Soviet spy in a mine. The terrified spy shoots Henry, who then kills the man and takes a mysterious briefcase. The play reveals that Henry, missing for 12 days, was found with a hand wound near a Nevada cave, and the Russian was later discovered dead inside. The briefcase, as revealed on stage, contains secrets about opening rifts to Dimension X and advanced technology.

Connecting the Dots

According to the play, the briefcase’s contents transported Henry to Dimension X, where he first met the Mind Flayer. Later, Dr. Brenner linked Henry to the Mind Flayer after finding his spyglass—the same one Holly uses in the show—in the caves.

Both volumes of Stranger Things season 5 are now streaming. Don’t miss the finale, and if you need a refresher, check out the breakdown of the season’s ending. Which season stands out as your favorite so far?