Why won’t Vecna go in the cave? The actual reason was hidden in the Stranger Things play

In episode four of Stranger Things season five, Vecna aka Henry aka One aka Mr Whatsit had a massive freak out over a cave. Max explained that “there was something about this cave, this memory” and he was “terrified” to venture in. This moment makes zero sense if you’ve only watched the Netflix show. But the reasoning why Vecna refuses to go in the cave was all explained in the Stranger Things: The First Shadow play.

The play only opened in London in November 2023, and New York in April 2025. So you’re forgiven if you’re not up-t0-date with this lore. But Stranger Things: The First Shadow is a prequel about Vecna/Henry/One/whoever, and set in 1959. In the play, Dr Brenner’s father experiments in Nevada on a mysterious realm called Dimension X.

Vecna falling into Dimension X (Image via Netflix)

This is the realm that we saw Eleven yeet Vecna into, during a flashback in season four of the Stranger Things TV show. It is not the same as the Upside Down, but has similar vibes. We don’t know entirely how it relates to the Upside Down yet.

In the play, some equipment from Dr Brenner’s dad’s lab ends up in a cave in Nevada. When Henry/Vecna is eight, he plays with a spyglass in the cave, and stumbles upon the equipment. He is (accidentally) teleported to Dimension X, and spends 12 terrifying hours there. The Mind Flayer (who apparently used to kick about in Dimension X) messes with Henry/Vecna.

What an image
(Image via Netflix)

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Once he returns to the Rightside Up, the experience has fundamentally changed him. His personality is way creepier. His blood type has changed, and now he has magical mind-y powers. Dr Brenner figures all this out, and later transfuses Henry’s blood into the kids he experiments on (including Eleven). In a way, Henry wandering into this cave is the cause of all the events of Stranger Things.

Because Henry/Vecna’s first trip to Dimension X was so awful, and he was so young, his mind suppresses the memory. This is similar to how Eleven can’t remember huge chunks of her childhood until she works through (some of) her trauma with Dr Brenner in season four.

She’s in her interior design era
(Image via Netflix)

This backstory explains why Vecna refuses to go into the cave in Stranger Things season five. The incident is imbedded deeply in his psyche, which is why the cave pops up along with his childhood house in the realm the kids call Camazotz. But he can’t quite recall what happened in the cave. That’s why he freaks out at the sight of the cave, and Max can move in.

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Featured image credit: Netflix