One of the biggest surprises in the opening four episodes of Stranger Things Season 5 was the unsettling mystery tied to Henry Creel’s past. Specifically, fans were left puzzled by one question: why is Henry absolutely terrified of the cave that appears in his memories?

We learn in episode 4 that Max has been trapped in Henry’s mind as part of a world of memories. She’s desperate to escape and at one point, actually gets pretty close. Vecna catches on, though, and Max takes off running.

She keeps going until she reaches a cave system. With nowhere else to go, she ducks inside and that’s when something strange happens. Henry doesn’t follow her. Later, Max tells Holly that Henry wasn’t just uneasy about the cave. He was terrified of it. That fear ends up protecting Max, turning the cave into the one place Henry refuses to enter.

At first, the show doesn’t really explain why. It’s just left hanging there in Part 1, which made a lot of fans scratch their heads. But Episode 6, “Escape from Camazotz,” finally starts connecting the dots.

Henry Creel’s Worst Memory Explained

Holly decides she’s done waiting around and heads out with Max to look for a way out of Henry’s mental prison. They travel past the cave and end up underground, where they accidentally walk straight into one of Henry’s most disturbing memories.

There’s blood everywhere. You can hear someone breathing heavily, panicked. They follow the sound and find a man badly injured and clutching a briefcase. For a second, it almost looks like he can see them. He freaks out, yells, “How did you find me?” and points a gun straight ahead.

But he’s not talking to Max or Holly. A young Henry steps into view, wearing his Boy Scouts uniform. He calmly explains that he heard screaming while exploring the caves and came to help. The man doesn’t buy it for a second. Instead, he shoots Henry in the hand.

When the man gets ready to fire again, Henry rushes him, trying to grab the gun. He can’t. And in a really brutal moment, Henry picks up a rock and kills the man.

Max and Holly run. Holly looks back just long enough to see Henry covered in blood, the man lying dead behind him. As Max starts clearing rocks from a collapsed wall, Holly notices something else — Henry opening the briefcase.

We never see what’s inside. Max calls Holly away before the reveal, and the moment cuts off, leaving fans wondering what exactly Henry found in that cave.

The show itself doesn’t spell it out, but The First Shadow stage play fills in the missing pieces.

Who Was the Man Henry Creel Killed in the Cave

According to the play, the man Henry killed was a rogue scientist hiding near Rachel, Nevada. He had stolen a strange metal briefcase filled with experimental tech connected to interdimensional research. The tech was tied to Project Rainbow — a secret Navy experiment from the 1940s that involved the USS Eldridge disappearing into another dimension. The only survivor of that experiment was the ship’s captain… who also happened to be Dr. Brenner’s father.

When Henry accidentally activates the device in the cave, everything goes wrong. He gets pulled into Dimension X for around twelve hours, making him the first human to ever come into contact with the First Shadow.

Here’s the big reveal: Henry wasn’t born with powers. The play suggests he got them after being infected by interdimensional particles. Those particles changed his blood and basically rewired him — the powers were a side effect.

That context makes the man’s paranoia make a lot more sense. He was hiding from the U.S. military, and to him, Henry wasn’t a harmless kid. He thought it was a setup. In reality, Henry was just lost — wrong place, wrong time.

What was inside the briefcase?

Although Stranger Things Season 5 never outright states it, the briefcase contained military-grade rift technology tied to Dimension X.

When Henry experiments with it, he is transported into the other dimension, where he encounters a shadowy presence—later revealed to be the Mind Flayer. During those twelve hours, Henry is infected with the Mind Flayer’s particles, permanently altering his body and sealing his fate.

That moment inside the cave didn’t just traumatize Henry; it created Vecna.

And that is why, years later, even inside his own mind, Henry Creel cannot bring himself to step back into the cave that changed everything.