One rainy day, Bill was riding his bicycle when the mail truck in front of him suddenly stopped. Bill didn't. The crash left him paralyzed from the chest down. His autonomy, or what's left of it, comes from voice controls that let him lower and lift the blinds in his room or adjust the angle of his motorized bed. For everything else, he relies on round-the-clock care.

Bill doesn't know Anne, who has Parkinson's disease; her hands shake when she tries to apply makeup or weed the garden. Neither of them know Stephen, who went blind in adulthood from a degenerative condition, and needs his sister to navigate the outside world. Imagining the three of them together sounds like the setup to a bad joke—a blind man, a tetraplegic, and Parkinson's patient walk into a bar. But their stories come together in a new documentary, I Am Human, which premieres today at the Tribeca Film Festival.

The film follows the trio as they pursue experimental brain treatments, cracking open their skulls to insert electrodes in the hope of regaining what is lost—movement, eyesight, control of their bodies—and reclaiming some sense of freedom. For each of them, the journey is as much medical as it is philosophical, one that gives them command of their biological reality. The documentary also looks at the promises of neurotechnology to push the limits of what humans could accomplish with chips in their brains.

Taryn Southern, the co-director of the film, says she began thinking about the brain just as shows like Black Mirror and Westworld, which play on the relationship between humans and technology, began their ascendency. She found herself fascinated by the ways science fiction reimagined the role machines could play in human evolution—not just improving alongside humans, but actually changing the human species. (Southern has her own intimate relationship to technology: She spent last year recording an album that was largely composed with AI.)

"There seemed to be this disconnect between the dystopian ideas that we see in those shows and what was actually happening in the real world," says Southern, who considers herself a techno-optimist.

Southern is not alone in her fascination with the topic. Other films premiering at the Tribeca Film Festival explore similar themes: Almost Human looks at the relationships between humans and the robots they create; Universal Machine, a short film, follows a confrontation between a woman and lifelike AI.

"There seemed to be this disconnect between the dystopian ideas that we see in those shows and what was actually happening in the real world." Director Taryn Southern

Hundreds of thousands of people around the world already have brain-computer interfaces, which scientists have been developing since at least the 1970s, in part thanks to funding from Darpa. Some experts predict that number will reach one million in the next decade as the science becomes more sophisticated. "Real life is unfolding and it's cooler than science fiction," says Elena Gaby, Southern's codirector.

But the inner-workings of our brains are still not well understood, and the real returns on this kind of neurotechnology are just beginning to emerge. There are a hundred billion neurons in the brain, each of them "as complicated as the city of Los Angeles" with about 500 trillion connections, says David Eagleman, a neuroscientist who appears in the film. Treatments like the ones Bill, Stephen, and Anne received are still largely experimental, without any guarantee of working.

"It's interesting that we can count our steps, count calories, sequence our genome, draw our blood, and measure our heart rate, but we have virtually no insight into our brains," says Bryan Johnson, the founder and CEO of neuroscience startup Kernel. "We have this sliver of self-introspection, but otherwise, it's a black box."

It's fear of the brain's great unknowns that separate what the subjects of I Am Human pursue from Big Idea sci-fi. Watching Bill, Stephen, and Anne grappling with the decision to implant chips in their brains is a far more difficult reality than anything in Black Mirror. "Someone's cutting into your brain," Anne says in the film. "You don't know what's going to happen."