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During the bulk of season 5 of *Stranger Things*, Holly Wheeler and Max Mayfield find themselves trapped in a realm of memories constructed from the memories of the villain Henry. Their mission for freedom in the pivotal sixth episode leads them to experience a traumatic incident from Henry's early years—an event that could contain the secret to his downfall once and for all.

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

A Journey Into the Cavern

After locating Max in the psychic realm known as Camazotz, Holly discovers she is taking refuge in a grotto that Henry is terrified of. An accidental discovery a spyglass taken from Henry's old home lets Holly see that the danger lies beyond the cave. When she lines up her view to the cut-out, she and Max suddenly fall into a deep mine shaft.

There, they witness a Henry as a boy find an man who is hurt and scared. As the boy offers aid, the man reacts violently, hitting him in the hand. To protect himself, Henry kills the man with a rock. "Is this what corrupted him?" Holly wonders aloud. The answer is affirmative, but the trauma involved more than a simple act of survival.

In keeping with mysteries of the final season, the full story behind the encounter is detailed in the theatrical production The First Shadow.

A Naval Mystery and Secret Science

*The First Shadow* opens with the infamous Philadelphia Experiment, which transported a warship into a dimension dubbed the Abyss. Demogorgons assaulted it, and the only one to return was the captain, who returned in a vegetative state with an different blood composition.

The captain's son, Martin Brenner, sought to replicate this dimensional breach in remote Nevada. But a disloyal lab worker took experimental components to sell to the Soviets, who were eager to begin their parallel research. The man in the cavern is the one who stole the materials.

Unlocking the Mystery

Based on what is understood of Brenner's work, the container almost certainly contained vials of Captain Brenner's infected blood. At that moment, Henry is wounded and bleeding. He likely becomes contaminated upon handling the contents, leading to the origin of his telekinetic gifts—traits he would genetically impart to the test subjects under Brenner's care.

A second theory, the box could have held exotic matter that later made its way to the Hawkins lab, where it helps stabilize the Upside Down. *The First Shadow* reveals that the boy somehow wound up in the Abyss, coming back after half a day as a different person. From then on, he fights the influence of a malevolent force, similar to how Will Byers was influenced by the entity in season 2.

Confronting Terror

Henry's enduring terror of that cave highlights how deeply scarring the incident was for him. While he orchestrates the Hive Mind and attempts to pull the Abyss into Hawkins, a part of him remains. Fear is his tool, and the narrative points to a showdown between the monster and Will, who has confronted his greatest terrors to become stronger.

This journey has effectively disarmed Vecna. Furthermore, Will now understands what his enemy truly fears, and this may grant a key upper hand in the endgame conflict that awaits.