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For much of the fifth and final season of the popular Netflix show, Holly Wheeler and Max are imprisoned in a realm of memories forged from the recollections of Vecna. Their quest for an escape route in episode 6 forces them to experience a life-altering moment from Henry's early years—an event that potentially carries the key to defeating him for good.

Heads Up: This article reveals crucial plot points for the concluding episodes.

A Journey Into the Mine Shaft

After locating Max in the mental landscape called Camazotz, Holly discovers she is taking refuge in a grotto that Henry strangely fears. An accidental discovery a viewing device pilfered from Henry's old home allows Holly to realize that the source of fear lies not within the cave. When she lines up her view to the cut-out, she and Max plummet into a subterranean passage.

There, they witness a child version of Henry come across an injured and paranoid man. Attempting assistance, the man reacts violently, wounding the boy in the hand. To protect himself, Henry kills the man with a rock. "Is this what corrupted him?" Holly asks tearfully. The answer is affirmative, but the event went deeper than a defensive killing.

In keeping with mysteries this season, the full story behind the mine shaft incident was first explored the live show the prequel play.

The Philadelphia Experiment and Secret Science

The play reveals the infamous Philadelphia Experiment, which transported a vessel into a dimension known as the Abyss. It faced creatures, and the lone returnee was the ship's commander, who emerged in a vegetative state with an different blood composition.

This man's son, a young Martin Brenner, attempted to recreate this experiment in the Nevada desert. One of his scientists pilfered lab samples to give to the Communist spies, who were desperate to launch their version of the tests. That traitor in the cavern was that very thief.

Unlocking the Mystery

Based on what is known about his research, the case would assuredly have vials of Captain Brenner's infected blood. As shown, Henry is bleeding from his hand. Exposure likely occurred upon opening the case, leading to the development of his psychic abilities—powers he would later pass on to the test subjects under Brenner's care.

A second theory, the box could have held strange particles that later made its way to Hawkins Laboratory, where it maintains the stability of the mirror dimension. The stage play explains that the boy somehow wound up in the Abyss, returning after half a day fundamentally changed. Throughout the play, he struggles with the control of a malevolent force, similar to how the character Will was possessed by the Mind Flayer in season 2.

The Power of Fear

The reason Henry avoids the cave linked to that memory highlights how profoundly traumatic the event was for him. Even as he controls the army of monsters and attempts to pull the Abyss into Hawkins, a part of him lingers. Vecna uses fear, and the series is building toward a showdown between Vecna and Will, who has deliberately faced his greatest terrors to become stronger.

His confrontation has effectively disarmed Vecna. Crucially, Will now is aware of what his enemy truly fears, which could provide a key upper hand in the endgame conflict to come.