The Contents of that Briefcase Henry Opens in Stranger Things?

During the bulk of the fifth and final season of the popular Netflix show, the young Holly Wheeler and Max are imprisoned within a psychic prison constructed from the past of the villain Henry. Their mission for a way out in the pivotal sixth episode has them relive a defining event from Henry's childhood—an event that potentially carries the means to overcome him for good.

Editor's Note: This discussion contains major spoilers for *Stranger Things* season 5, volume 2.

The Discovery in the Cavern

After locating Max in the mental landscape called Camazotz, it becomes clear she is taking refuge in a grotto that Henry strangely fears. By leaving the lens cap on a telescope taken from Henry's old home helps Holly understand that the danger lies outside of the cave. By aligning her view to the cut-out, she and Max tumble into a abandoned tunnel.

There, they witness a child version of Henry find an injured and paranoid man. Attempting assistance, the man panics and shoots, hitting him in the hand. To protect himself, Henry strikes the man down with a rock. "Is this what corrupted him?" Holly wonders aloud. It was a factor, but the trauma went deeper than a fight for life.

In keeping with mysteries of the final season, the truth about the mine shaft incident originates in the live show the prequel play.

The Philadelphia Experiment and Secret Science

The play reveals the legendary naval test, which reportedly teleported a warship into a realm later called the Abyss. Demogorgons assaulted it, and the only one to return was the captain, who came back in a vegetative state with an different blood composition.

The captain's son, the future Dr. Brenner, sought to replicate this phenomenon in the Nevada desert. However, a treacherous employee stole research materials to give to the Communist spies, who were keen to start their version of the tests. That traitor in the mine shaft is the one who stole the materials.

Unlocking the Mystery

Given what is revealed about his research, the case almost certainly contained vials of Captain Brenner's infected blood. As shown, Henry is wounded and bleeding. It's probable he was infected upon contact with the blood, leading to the development of his psychic abilities—abilities he would inadvertently transfer to the children Brenner would experiment on.

A second theory, the case could have held dark matter that later made its way to the Hawkins lab, where it maintains the stability of the mirror dimension. The stage play explains that Henry somehow wound up in the Abyss, coming back after half a day with a new blood type and personality. Throughout the play, he fights the control of a sinister presence, much like how Will Byers was possessed by the entity in an earlier season.

The Power of Fear

The reason Henry avoids the cave linked to that memory highlights how deeply scarring the incident was for him. Even as he commands the Hive Mind and seeks to merge the Abyss into Hawkins, a fragment of his psyche lingers. Vecna uses fear, and the narrative points to a showdown between Vecna and Will, who has confronted his own deepest fears to fortify himself.

This journey has taken a weapon from Vecna. Crucially, Will now understands what the monster's own weakness, which could provide a decisive edge in the series finale to come.