Computer-generated videos are getting more realistic and even harder to detect thanks to deep learning and artificial intelligence. As WSJ’s Jason Bellini finds in this episode of Moving Upstream, these so-called deepfakes can be playful, but can also have real, damaging consequences for people’s lives.

FOR WEEKS, COMPUTER scientist Siwei Lyu had watched his team’s deepfake videos with a gnawing sense of unease. Created by a machine learning algorithm, these falsified films showed celebrities doing things they'd never done. They felt eerie to him, and not just because he knew they’d been ginned up. “They don’t look right,” he recalls thinking, “but it’s very hard to pinpoint where that feeling comes from.”

Doctored photos can easily create false memories. What happens when there’s fake video?