Breaking Bad took fans on a wild five-season journey through the New Mexico underworld with science-teacher-turned-drug-kingpin Walter White. But when did Walt truly become Heisenberg? What was the exact moment the change happened?

In our latest video essay, the ScreenCrush team pinpoints the most important episode in the history of the show: “Gray Matter,” from Breaking Bad’s first season. Written by Patty Lin and directed by Tricia Brock, the episode focuses on a hugely crucial moment in Walter’s life, when he decides to accept treatment for his cancer — and to plunge deeper into his potential life of crime. Watch exactly what happens, and why it’s so important, below:

If you liked this video on the most important episode in the history of Breaking Bad, check out some more of our videos below, including all of the Breaking Bad references in El Camino, our video recap of every episode of the series in just 15 minutes, and a collection of Breaking Bad trivia from the show’s heyday. Plus, there’s tons more over at ScreenCrush’s YouTube channel. Be sure to subscribe to catch all our future episodes. El Camino is available on Netflix now.

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