When the United States' National Security Agency's code-breaking machine encounters a mysterious and ingeniously complex code called Digital Fortress that it cannot break, the agency calls in Susan Fletcher, a brilliant mathematician and their head cryptographer, to crack it. Fletcher eventually discovers that the code is a viral program designed to break down the NSA's firewall and encryption systems, allowing anyone anywhere to access all of the NSA's files. She also discovers that it was written by Ensei Tankado, a former NSA employee who became displeased with the NSA's intrusion into people's private lives. Tankado, essentially holding the NSA hostage, intends to auction the code's algorithm on his website, and release it for free if he dies, threatening to cripple U.S. intelligence. Fletcher, along with her fiancé, David Becker, a skilled linguist with eidetic memory, are thrust into a global chase that takes them from Washington, DC to the towers of Tokyo to the cathedrals of Seville, Spain, where they must find a way to stop the spread of the code.