As the conflict in Vietnam heats up, Simon Hannay is pursuing his Masters in Comparative Literature at a Midwest university, teaching karate on the side and doing his best to avoid the draft. He's not overly excited about his thesis... until he stumbles upon an encrypted renaissance journal that's been hidden for decades in the rare book room.

Simon, who's been fascinated by codes and ciphers since he was a kid, convinces his advisor to let him use the mysterious codex as his thesis topic. But first he has to decipher it—and, as he soon learns, that's going to be a formidable task. To add to his problems, an unidentified rival he calls the Mystery Man is desperate to get his hands on the codex—so desperate that he ransacks Simon's apartment and even drugs and interrogates him and his advisor.

With the help of Gabriela, a Brazilian classmate, Simon finally hits upon the key to the cipher. Together they struggle to unravel and translate the text, which was penned by a 16th-century Portuguese fortune-hunter. In the process, they learn the secrets the codex contains, and why the Mystery Man is so determined to steal it.