Stranger Things Season 5 Unveils the Truth About the Upside Down After Nearly a Decade, Hinting at Clues from the 2023 Stage Show

After nearly a decade of speculation, Stranger Things has finally unveiled the true nature of the Upside Down. As Dustin Henderson points out in season 5, all our previous theories about this dark realm have been “dead wrong”—unless, of course, you caught the stage show, Stranger Things: The First Shadow.

**Spoiler Alert!** Major plot details for Stranger Things season 5 follow.

In the opening of Stranger Things season 5 Vol. 2, Nancy, Jonathan, Steve, and Dustin decide to ignore Hopper’s orders to stay put at the church. Instead, they venture into the Upside Down to investigate the massive wall they crashed into. Dustin theorizes that this wall might be a dark magic shield created by Vecna, similar to the Empire’s energy shield around the Death Star II in Star Wars: Return of the Jedi. If he’s correct, they could locate the shield’s generator, disable it, and infiltrate Vecna’s lair to rescue Holly.

Their quest leads them to Hawkins Lab, where Dustin discovers Dr. Martin Brenner’s old research. He learns that the Upside Down is actually a wormhole—or interdimensional bridge—connecting their world to a mysterious realm they call ‘The Abyss.’ This dimension is likely the origin of the demogorgons, demobats, and the Mind Flayer, as well as the place where Eleven trapped Henry Creel/One during their psychic showdown in the lab’s Rainbow Room back in 1979.

Henry was stuck there for four years until Eleven made contact with a demogorgon, which created the tunnel to the Upside Down. Since then, Henry/One/Vecna has been using this realm as a gateway to the real world, crafting gates to destabilize it and ultimately merge the Abyss with Hawkins. In essence, he aims to transform Earth into a monstrous hellscape.

Matt Duffer recently shared that Netflix wanted clarity on the show’s mythology, as the creators were keen on maintaining an air of mystery. “We wrote a 20-page mythology document,” he explained. Initially, they referred to the Abyss as Dimension X, a nod to the Ninja Turtles. After holding back this information for so long, it felt great to finally reveal it.

If you’ve seen The First Shadow, you might already be familiar with ‘Dimension X.’ The play, which delves into Henry’s backstory, begins in 1943 with the USS Eldridge being pulled into Dimension X. This ship was captained by Brenner, Martin’s father, who was the sole survivor of the incident, with his crew taken out by sinister smoky entities.

While the play doesn’t mention that Joyce and Hopper attended school with Henry—a significant plot point hinted at in Vol. 1—Vol. 2 includes a scene that will resonate more with those who have seen the stage show. During their exploration of Vecna’s Mind Lair, Max and Holly stumble upon a traumatic memory where a young Henry, dressed in a Boy Scout uniform, encounters a bloodied Soviet spy in a mine shaft.

In a panic, the spy shoots Henry in the hand, leading the boy to fatally strike the man with a rock and seize a metallic briefcase he had stolen from the US military. The play reveals that Henry, who had been missing for 12 days, was found with a hand injury near a cave system in Nevada, while the Russian was later discovered dead inside the caves. Although the Netflix series doesn’t disclose the briefcase’s contents, those familiar with the play know it holds rift-opening secrets related to Dimension X and other classified tech.

The play establishes that the briefcase’s contents transported Henry to Dimension X, where he first encountered the Mind Flayer. Later, Dr. Brenner connected the dots between Henry and the Mind Flayer after discovering Henry’s spyglass, which Holly uses in the show, in the caves.

Stranger Things season 5 Vol. 1 and Vol. 2 are currently available for streaming.