In the days before anesthesia, surgery was about the worst ordeal you could endure. Patrick Purdon, assistant professor of anesthesia at Harvard Medical School, gives producer Tim Howard a tour of Mass General Hospital’s famous Ether Dome, an operating theater that would have resonated with the screams of patients on a daily basis in the early 1800s. Writer Julie Fenster introduces us to a con artist-turned-dentist named William Morton, the man who would become famous in 1846 (if undeservedly, as contemporaries would claim) for the discovery of painless surgery.

With the discovery of anesthesia, however, came a strange new problem: nobody was quite sure what was happening when the brain slipped from consciousness into this new, drugged state. We meet Carl Zimmer, whose own experience with anesthesia leads him to wonder how it’s possible that time seems to simply disappear when you go under. What happens in that invisible moment? And why is it that some patients remain conscious, even when they appear to be knocked out?

Patrick Purdon tries to answer this question in an experiment that takes the induction of anesthesia and slows it down to a crawl while analyzing the brain’s electrical activity. With the help of his colleague Emery Brown, professor of anesthesia at Harvard Medical School, Patrick explains the strange electrical signature they discovered, and why it just may be that sought-after indicator of when a brain is truly, totally, definitely unconscious.