The Contents of that Case Henry Opens in Stranger Things?
During the bulk of the concluding season of the popular Netflix show, the young Holly Wheeler and Max Mayfield are imprisoned inside a mental fortress constructed from the memories of Vecna. Their quest for an escape route in "Escape from Camazotz" has them experience a defining event from Henry's youth—an event that may just hold the secret to his downfall once and for all.
Spoiler Warning: The following details significant events for the concluding episodes.
A Journey Into the Cavern
After locating Max in the mental landscape called Camazotz, Holly discovers she is sheltering in a cave that Henry strangely fears. By leaving the lens cap on a viewing device taken from Henry's old home lets Holly see that the true terror lies outside of the cave. By aligning her view through the cap, she and Max tumble into a abandoned tunnel.
There, they witness a young, eight-year-old Henry encounter an injured and paranoid man. Attempting assistance, the man reacts violently, hitting him in the hand. To protect himself, Henry ends the threat with a rock. "Was this the turning point?" Holly wonders aloud. It was a factor, but the event went deeper than a defensive killing.
Similar to other plot points this season, the truth about the encounter is detailed in the stage play The First Shadow.
An Interdimensional Incident and Secret Science
This backstory explains the infamous Philadelphia Experiment, which transported a warship into a plane later called the Abyss. Demogorgons assaulted it, and the lone returnee was the captain, who returned in a vegetative state with an altered blood type.
That survivor's offspring, the future Dr. Brenner, attempted to recreate this experiment in the Nevada desert. However, a treacherous employee pilfered lab samples to sell to the Communist spies, who were eager to begin their parallel research. This spy in the cavern was that very thief.
Unlocking the Mystery
Based on what is understood of his research, 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 contact with the blood, causing the development of his psychic abilities—powers he would later pass on to the test subjects in Hawkins Lab.
A second theory, the box could have held dark matter that eventually found its way to Hawkins Laboratory, where it helps stabilize the mirror dimension. The prequel shows that young Henry ended up in the Abyss, returning hours later with a new blood type and personality. As a youth, he battles the control of a malevolent force, much like how the character Will was influenced by the entity in the past.
A Weaponized Emotion
Henry's enduring terror associated with the mine demonstrates how profoundly traumatic the encounter was for him. Even as he commands the Hive Mind and seeks to merge the Abyss into Hawkins, a fragment of his psyche remains. Vecna uses fear, and the narrative points to a final confrontation between him and Will, who has confronted his personal demons to fortify himself.
This journey has neutralized a key tactic of Vecna. Crucially, Will now understands what Vecna himself fears, potentially giving him a critical advantage in the endgame conflict to come.
