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For much of the concluding season of the popular Netflix show, Holly Wheeler and Max are held captive inside a mental fortress constructed from the past of Vecna. Their mission for an escape route in the pivotal sixth episode leads them to witness again a defining event from Henry's early years—an event that could contain the means to overcome him once and for all.

Spoiler Warning: This discussion details significant events for the second part of the final season.

The Discovery in the Tunnel

When she finds Max in the world called Camazotz, Holly discovers she is sheltering in a cave that Henry is terrified of. An accidental discovery a telescope found in Henry's old home helps Holly understand that the danger lies beyond the cave. By aligning her view with the shape, she and Max plummet into a subterranean passage.

There, they witness a young, eight-year-old Henry find an wounded, fearful individual. When Henry tries to help, the man reacts violently, wounding the boy in the hand. Fighting for his life, Henry strikes the man down with a rock. "Is this what corrupted him?" Holly asks tearfully. Essentially, yes, but the event went deeper than a defensive killing.

In keeping with mysteries this season, the truth about the encounter is detailed in the theatrical production the prequel play.

The Philadelphia Experiment and Stolen Research

This backstory explains the 1943 event, which transported a vessel into a dimension later called the Abyss. The ship was attacked, and the only one to return was the ship's commander, who emerged in a unresponsive state with an altered blood type.

This man's son, a young Martin Brenner, sought to replicate this experiment in the Nevada desert. But a disloyal lab worker pilfered lab samples to give to the Communist spies, who were desperate to launch their own interdimensional projects. The man in the cavern was that very thief.

Unlocking the Mystery

Considering what is understood of the doctor's experiments, the case would assuredly have vials of the unique blood type. In the scene, Henry is bleeding from his hand. He likely becomes contaminated upon contact with the blood, leading to the development of his psychic abilities—powers he would later pass on to the children Brenner would experiment on.

Alternatively, the container could have held dark matter that was destined to travel to the secret facility, where it helps stabilize the mirror dimension. The stage play explains that young Henry ended up in the Abyss, coming back 12 hours later with a new blood type and personality. As a youth, he struggles with the whispers of a malevolent force, much like how Will was controlled by the Mind Flayer in season 2.

Confronting Terror

The reason Henry avoids the cave linked to that memory demonstrates how utterly horrifying the event was for him. While he controls the collective consciousness and works to collapse the Abyss into Hawkins, a part of him lingers. Vecna uses fear, and the narrative points to a showdown between him and Will, who has deliberately faced his personal demons to become stronger.

This journey has effectively disarmed Vecna. Crucially, Will now knows what Vecna himself fears, which could provide a decisive edge in the ultimate battle that awaits.