“It was a simple apocalypse story,” showrunner Charlie Brooker explained in a 2016 interview. “It was a woman who wakes up and there’s a sign that goes around the world and affects people. The original ending had a public crucifixion.” The idea for the ending we’re all familiar with didn’t occur to Brooker until after they started scouting locations for the episode, which involved looking around a former US Air Force base. As Brooker said:
“We were looking around to see what we could use… I saw the house, a gas station, and I saw this fence running out of the place. I thought, ‘That’s interesting, there’s a fence. A fence. A fence!’ And all of a sudden the twist occurred to me. I ran out and wrote it in a couple of days in a fever dream.”
It’s a surprising explanation, because it’s hard to rewatch the first two-thirds of the episode and not feel like it was all written with the twist in mind. While Brooker of course rewrote the entire episode and not just added the ending, the consistency of it all still feels like a miracle. The episode is incredibly chaotic at first glance, but it always feels like controlled chaos. It’s never as messy as productions with last-minute changes tend to be.