The idea took some time to progress from stray thought to actual effort, but once it did Engelbrecht and Yellin knew just where to start. Netflix and DreamWorks had expanded their partnership in 2015 to include original content; why not explore the idea in a kids' show first?

"We felt that if it didn't succeed in the kids' space, it wouldn't succeed with grownups," Yellin says now. "Kids don't know how something's supposed to be, they just know how it is." Besides, the company's research and testing made them confident kids would be that much more willing to buy in: "Not only do they want to get involved with characters, but they want to dive in and be with the characters."

So Yellin and Engelbrecht (who is now Netflix's director of product innovation) met with the showrunners of Puss in Boots and a few other shows, and brainstormed their way into a quartet of standalone specials that featured branching narratives. You'd get to a certain point in the story, and a binary prompt would appear on the screen, giving you 10 seconds or so to decide via remote control or tapping the screen. Should Puss kiss Dulcinea or shake her hand? Should Buddy and Darnell from Buddy Thunderstruck have a wet willie contest, or should they work out and "get jacked"?

"Kids in general don't hesitate to talk to the screen," Engelbrecht says. "But they immediately lit up. One of my absolutely favorite moments was with an 8- and 9-year-old brother and sister. They'd go back and forth: 'Shake hands!' 'Kiss!' They'd cheer and boo, and all all the while had these smiles on their faces—and their mom too."

The kids' shows were always meant as a test run, of course—and before they even aired, Yellin and Engelbrecht had decided who they wanted to create the live-action, adult-oriented version. In 2015, Netflix had put a ring on Black Mirror, transforming it from a catalog show to a Netflix-produced one. Creator Charlie Brooker and executive producer Annabel Jones had already proven they could draw an audience eager for something new; now, Yellin and Engelbrecht just had to convince them of a brand-new something new.

In May 2017, the duo—along with Netflix's head of interactive design—met with Brooker and Jones in Los Angeles. "We were so primed to get them excited," Yellin says. "We gave them the slideshow, the whole rollout." The response? "They seemed … OK with it."

"I think we did basically walk out of the room and go, 'no,'" says Brooker. He immediately thought back, he says, to the slow, maddening CD-ROM videogames of the '90s: "I thought, 'Well, they're often really clunky and I can't see how that's going to translate into Black Mirror.'" So that was that.

"It's the only thing I've ever worked on where the story treatment crashed." Black Mirror creator Charlie Brooker

Except it wasn't. Some weeks later, in a story meeting, an idea popped up that Brooker and Jones realized would only work if it was interactive: a videogame designer in the 1980s, trying to adapt a choose-your-own-adventure novel (which, you may remember, was the style at the time). Oh, shit, Brooker thought. Now we've got to do that, and that's probably going to be complicated.

He was more right than he knew. They started plotting out the story on a whiteboard. "I figured it was going to be pretty much regular," he says, "with a few bits changed at the end." But then they hit on a new wrinkle: What if the story could remember what you'd chosen at earlier points, and incorporate references to those earlier choices? "As soon as that happened," Brooker says, "it was clear that we had something more complicated than a flowchart."

The episode Brooker and Jones kicked back to Netflix wasn't a script in any conventional sense of the word: It was essentially a vast, sprawling outline written in the videogame programming language Twine, which Brooker had taught himself because it was the only way to capture the intra-linked complexity of all the various tributaries and recursions of the Bandersnatch story. "Every time I had an idea I put it in a box, and you can move them around. It's a bit like making a giant patchwork quilt," he says.