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Throughout a large portion of season 5 of the popular Netflix show, the young Holly Wheeler and Max Mayfield are held captive within a psychic prison constructed from the memories of Vecna. Their quest for a way out in the pivotal sixth episode has them witness again a life-altering moment from Henry's childhood—a moment that could contain the key to defeating him once and for all.

Spoiler Warning: The following details significant events for the second part of the final season.

The Discovery in the Tunnel

When she finds Max in the mental landscape dubbed Camazotz, Holly discovers she is taking refuge in a cave that Henry strangely fears. An accidental discovery a viewing device taken from Henry's old home helps Holly understand that the true terror lies not within the cave. When she lines up her view through the cap, she and Max suddenly fall into a deep mine shaft.

They watch a child version of Henry come across an injured and paranoid man. When Henry tries to help, the man reacts violently, wounding the boy in the hand. To protect himself, Henry strikes the man down with a rock. "Was this the turning point?" Holly asks tearfully. Essentially, yes, but the trauma involved more than a defensive killing.

Like many elements of the final season, the significance of the events in the tunnel originates in the live show the prequel play.

The Philadelphia Experiment and A Spy's Mission

The play reveals the legendary naval test, which allegedly sent a vessel into a plane later called the Abyss. It faced creatures, and the only one to return was the ship's commander, who came back in a catatonic condition with an different blood composition.

This man's son, a young Martin Brenner, wanted to repeat this dimensional breach in remote Nevada. One of his scientists took experimental components to give to the Communist spies, who were desperate to launch their own interdimensional projects. This spy in the tunnel is that employee.

Unlocking the Mystery

Considering what is understood of his research, the briefcase almost certainly contained some of the unique blood type. In the scene, Henry is bleeding from his hand. He likely becomes contaminated upon opening the case, causing the development of his psychic abilities—traits he would genetically impart to the kids in Hawkins Lab.

A second theory, the container could have held dark matter that eventually found its way to Hawkins Laboratory, where it acts to anchor the parallel world. *The First Shadow* reveals that Henry somehow wound up in the Abyss, returning after half a day as a different person. From then on, he fights the whispers of a dark entity, reminiscent of how Will Byers was influenced by the Mind Flayer in season 2.

Confronting Terror

Henry's enduring terror of that cave demonstrates how deeply scarring the event was for him. Even as he commands the Hive Mind and works to collapse the Abyss into Hawkins, a part of him lingers. Fear is his tool, and the narrative points to a final confrontation between him and Will, who has deliberately faced his greatest terrors to fortify himself.

Will's growth has taken a weapon from Vecna. More importantly, Will now is aware of what the monster's own weakness, which could provide a key upper hand in the series finale to come.