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For much of season 5 of *Stranger Things*, Holly Wheeler and Max Mayfield find themselves trapped within a psychic prison forged from the memories of Vecna. The search for a way out in the pivotal sixth episode forces them to witness again a defining event from Henry's youth—a discovery that may just hold the secret to his downfall for good.

Spoiler Warning: This article reveals crucial plot points for the second part of the final season.

The Discovery in the Cavern

When she finds Max in the mental landscape called Camazotz, Holly discovers she is sheltering in a chamber that Henry strangely fears. By leaving the lens cap on a spyglass taken from Henry's old home helps Holly understand that the danger lies beyond the cave. Matching her view through the cap, she and Max tumble into a deep mine shaft.

They watch a child version of Henry come across an injured and paranoid man. As the boy offers aid, the man reacts violently, striking Henry in the hand. In a desperate act of self-defense, Henry kills the man with a rock. "Was this the turning point?" Holly wonders aloud. Essentially, yes, but the event wasn't just a fight for life.

In keeping with mysteries in volume 2, the truth about the encounter was first explored the live show Stranger Things: The First Shadow.

The Philadelphia Experiment and Secret Science

The play reveals the legendary naval test, which allegedly sent a warship into a realm known as the Abyss. The ship was attacked, and the lone returnee was the captain, who emerged in a vegetative state with an altered blood type.

This man's son, Martin Brenner, sought to replicate this phenomenon in Nevada. But a disloyal lab worker pilfered lab samples to sell to the Soviets, who were desperate to launch their own interdimensional projects. This spy in the tunnel is the one who stole the materials.

The Briefcase's Contents

Given what is understood of the doctor's experiments, the briefcase most likely held vials of Captain Brenner's infected blood. At that moment, Henry is already bleeding. It's probable he was infected upon contact with the blood, which triggered the development of his psychic abilities—powers he would inadvertently transfer to the test subjects Brenner would experiment on.

A second theory, the container could have held exotic matter that later made its way to the Hawkins lab, where it acts to anchor the mirror dimension. The stage play explains that the boy ended up in the Abyss, reappearing after half a day as a different person. As a youth, he battles the control of a malevolent force, much like how Will was influenced by the entity in an earlier season.

Confronting Terror

The reason Henry avoids the cave associated with the mine highlights how profoundly traumatic the encounter was for him. Even as he orchestrates the collective consciousness and attempts to pull the Abyss into Hawkins, a piece of the scared boy remains. The villain weaponizes terror, and the show is setting up a final confrontation between him and Will, who has overcome his personal demons to fortify himself.

This journey has effectively disarmed Vecna. More importantly, Will now understands what the monster's own weakness, which could provide a key upper hand in the ultimate battle that awaits.