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For much of season 5 of the popular Netflix show, the young Holly Wheeler and Max Mayfield are held captive inside a mental fortress forged from the past of the villain Henry. Their mission for an escape route in the pivotal sixth episode leads them to witness again a traumatic incident from Henry's youth—an event that may just hold the key to defeating him once and for all.

Editor's Note: This article contains major spoilers for the second part of the final season.

Unearthing the Secrets of the Cavern

After locating Max in the psychic realm called Camazotz, Max reveals she is taking refuge in a grotto that Henry strangely fears. An accidental discovery a viewing device found in Henry's old home helps Holly understand that the true terror lies beyond the cave. By aligning her view through the cap, she and Max tumble into a deep mine shaft.

In this place, they observe a Henry as a boy come across an injured and paranoid man. Attempting assistance, the man reacts violently, hitting him in the hand. Fighting for his life, Henry ends the threat with a rock. "Did this make Henry evil?" Holly questions through tears. Essentially, yes, but the trauma went deeper than a defensive killing.

Like many elements in volume 2, the significance of the events in the tunnel was first explored the stage play the prequel play.

An Interdimensional Incident and Stolen Research

This backstory explains the infamous Philadelphia Experiment, which reportedly teleported a warship into a plane later called the Abyss. It faced creatures, and the only one to return was the captain, who emerged in a catatonic condition with an entirely new blood.

That survivor's offspring, Martin Brenner, sought to replicate this dimensional breach in remote Nevada. But a disloyal lab worker stole research materials to sell to the Russian agents, who were keen to start their version of the tests. This spy in the cavern is the one who stole the materials.

The Briefcase's Contents

Based on what is revealed about Brenner's work, the briefcase almost certainly contained vials of Captain Brenner's infected blood. As shown, Henry is bleeding from his hand. He likely becomes contaminated upon contact with the blood, which triggered the awakening of his powers—abilities he would inadvertently transfer to the children under Brenner's care.

A second theory, the box could have held strange particles that eventually found its way to the secret facility, where it maintains the stability of the mirror dimension. The stage play explains that the boy somehow wound up in the Abyss, coming back 12 hours later as a different person. As a youth, he fights the control of a sinister presence, much like how Will was possessed by the shadow monster in season 2.

The Power of Fear

Henry's enduring terror of that cave highlights how deeply scarring the event was for him. Even as he controls the army of monsters and works to collapse the Abyss into Hawkins, a piece of the scared boy remains. Fear is his tool, and the narrative points to a climactic battle between him and Will, who has confronted his own deepest fears to gain strength.

His confrontation has taken a weapon from Vecna. More importantly, Will now is aware of what his enemy truly fears, potentially giving him a key upper hand in the series finale that awaits.