On January 4, 2012, an anonymous post appeared on 4chan containing an image embedded with a hidden cryptographic puzzle and a challenge to solve it. This short documentary follows the trail that puzzle set off: layers of steganography, references to the Mayan calendar and the Book of Mormon, and clues that pushed solvers off their computers and into the physical world, where posters with QR codes turned up taped to lampposts in cities including Seattle, Warsaw, and Sydney. Interviews with amateur codebreakers and security researchers who chased the puzzle track how a small group cracked early layers only to hit the Liber Primus, a still partly unsolved manuscript written in runes. The film lays out the competing theories about who built Cicada 3301 and why, from a private cryptography contest to an intelligence-agency recruitment tool, without settling on one. What holds the story together is the obsession itself, the way an anonymous image convinced strangers around the world to spend months of their lives on a puzzle with no stated prize.