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For much of the concluding season of the popular Netflix show, Holly Wheeler and Max are imprisoned inside a mental fortress forged from the past of Vecna. The search for a way out in the pivotal sixth episode forces them to relive a defining event from Henry's childhood—a discovery that may just hold the key to defeating him for good.

Heads Up: This article contains major spoilers for the second part of the final season.

A Journey Into the Cavern

Upon meeting Max in the psychic realm known as Camazotz, Holly discovers she is taking refuge in a chamber that Henry avoids at all costs. By leaving the lens cap on a telescope found in Henry's old home helps Holly understand that the source of fear lies outside of the cave. When she lines up her view to the cut-out, she and Max tumble into a subterranean passage.

There, they witness a child version of Henry find an injured and paranoid man. When Henry tries to help, the man reacts violently, hitting him in the hand. To protect himself, Henry ends the threat with a rock. "Did this make Henry evil?" Holly wonders aloud. The answer is affirmative, but the event wasn't just a fight for life.

In keeping with mysteries of the final season, the truth about the events in the tunnel is detailed in the live show The First Shadow.

The Philadelphia Experiment and A Spy's Mission

This backstory explains the 1943 event, which reportedly teleported a vessel into a realm known as the Abyss. Demogorgons assaulted it, and the only one to return was the captain, who returned in a vegetative state with an altered blood type.

That survivor's offspring, Martin Brenner, wanted to repeat this experiment in remote Nevada. One of his scientists stole research materials to sell to the Russian agents, who were desperate to launch their own interdimensional projects. This spy in the cavern was that very thief.

The Briefcase's Contents

Given what is known about the doctor's experiments, the briefcase almost certainly contained samples of Captain Brenner's infected blood. In the scene, Henry is wounded and bleeding. It's probable he was infected upon contact with the blood, which triggered the development of his psychic abilities—abilities he would later pass on to the test subjects Brenner would experiment on.

Another possibility, the case could have held strange particles that was destined to travel to the Hawkins lab, where it acts to anchor the Upside Down. *The First Shadow* reveals that young Henry was transported in the Abyss, returning after half a day as a different person. From then on, he struggles with the whispers of a malevolent force, similar to how Will was influenced by the entity in season 2.

A Weaponized Emotion

The reason Henry avoids the cave of that cave demonstrates how utterly horrifying the incident was for him. Even as he orchestrates the Hive Mind and seeks to merge the Abyss into Hawkins, a fragment of his psyche lingers. The villain weaponizes terror, and the series is building toward a climactic battle between the monster and Will, who has overcome his greatest terrors to fortify himself.

His confrontation has neutralized a key tactic of Vecna. More importantly, Will now is aware of what Vecna himself fears, and this may grant a critical advantage in the endgame conflict to come.