Between 1968 and 1974, an unidentified killer in Northern California taunted police and newspapers with cipher letters signed with a crossed-circle symbol, claiming credit for murders around Vallejo and San Francisco and boasting he would never be caught. This documentary revisits the case through interviews with retired detectives, journalists, and codebreakers who worked the files for decades, including Robert Graysmith, whose research became the basis for later dramatizations. Archival news footage, crime scene photographs, and the killer's own letters are laid out alongside the competing suspect theories that have circulated for years, from a Vallejo garage mechanic to a fringe candidate favored by amateur sleuths online. The film gives real weight to the unsolved ciphers, showing how amateur and professional cryptographers alike have chased the unbroken 340-character message. It treats the case less as a whodunit with a tidy answer than as a study of how an unsolved crime metastasizes into folklore, obsession, and decades of competing claims about who the Zodiac really was.