At seven minutes past eight in the morning, on April 12, 1975, a sleeping man moved his eyes left, right, left, right, and became the first person in history to send a message out of a dream.

He was asleep. Genuinely asleep, deep in the stage where dreams happen, his body limp and his brain showing every sign of a mind gone off into the night. And yet, on the machine beside the bed, a pen traced a clean, deliberate, unmistakable pattern. Left, right, left, right. Not the random flicker of ordinary dreaming eyes. A code. Agreed in advance. Sent on purpose. A knock on the wall from inside a locked room.

Let me tell you how that room got knocked on, because it's the beginning of everything this season is about.

The dreaming brain is, for most of human history, a sealed room. Someone lies down, their eyes close, and they vanish into an inner world you cannot see, cannot enter, cannot verify. In the morning they tell you a story about it, and you take their word, because what else can you do? Every dream anyone has ever reported is hearsay. The dreamer walks out of the room and describes it, and the door has already closed behind them. There was never any way to check the report against the thing itself.

Worse, for the people who claimed a specific strange ability, science had a wall waiting. Some people said that sometimes, inside a dream, they woke up. Not woke up in bed, woke up inside the dream itself, suddenly aware "this is a dream," able to think clearly, look around, even decide what to do next, all while still asleep and still dreaming. It sounds like a contradiction, and for a long time scientists treated it as one. You are either awake or asleep. Consciousness is the thing that switches off when you dream. A person claiming to be lucidly, deliberately aware inside a dream was, to a sleep researcher, describing something like a square circle.

The problem was always the same sealed room. Even if you believed them, how could they prove it? By the time they woke and said "I was aware in there," they were awake, and it was hearsay again.

Keith Hearne, a researcher at the University of Hull in England, saw the crack in the wall. Here is the elegant piece of it. When you dream, your body is paralyzed, a natural safety catch that stops you from acting out your dreams. Almost every muscle is switched off. Almost. The eyes are not. In dreaming sleep the eyes move, that's the famous rapid eye movement, REM. So Hearne reasoned: if a dreamer is genuinely conscious inside the dream, and if the eyes are the one part of the body that still works, then a lucid dreamer could look left and right in the dream, in a pattern agreed beforehand, and their real eyes, wired to a recorder, would trace that same pattern in the waking world.

A message. In a channel that stays open while everything else is locked.

He worked with an experienced lucid dreamer named Alan Worsley, a man who could reliably become aware inside his dreams. They agreed on a signal: on becoming lucid, Worsley would sweep his eyes left and right, back and forth, in a deliberate sequence. Then Worsley went to sleep in the lab, wired up, while Hearne watched the pens.

I wasn't in that room, and the small human parts, the cold English morning, the coffee, the long boring hours of a man watching a needle and hoping, those I'm imagining. The trace is not imagined. In the early morning of April 12, 1975, deep in confirmed dreaming sleep, Worsley's eyes made the pattern. Left, right, left, right, exactly as agreed. Hearne watched a conscious, deliberate signal arrive from inside a dreaming brain, timestamped 8:07 in the morning.

The sealed room had been opened. Not much. A crack under the door, a single prearranged phrase in eye-movement Morse. But for the first time ever, a message had crossed from the dream world to ours while the dream was still happening, and it could be checked, timestamped, proven.

Everything else in this season builds on that dawn. But Hearne and Worsley were not the first people to notice you could wake up inside a dream. Not by a long way. A French aristocrat had been doing it, and writing it down in secret, more than a hundred years earlier.

Next episode: the man who directed his dreams.