Susan Fletcher, a brilliant mathematician and head of the National Security Agency's (NSA's) cryptography division, finds herself faced with an unbreakable code named "Digital Fortress", that TRANSLTR cannot break. TRANSLTR is the NSA's 3 million processor supercomputer that can crack encrypted data using the brute force method in a short amount of time, with some codes broken in more than 3 hours, others within minutes. The code of "Digital Fortress" is written by Japanese cryptographer Ensei Tankado, a fired employee of the NSA, who is displeased with the agency's intrusion into people's privacy. Tankado posts a copy of Digital Fortress on his website, encrypted with itself. Tankado auctions the passkey to unlock the algorithm on his website, threatening that his accomplice "North Dakota" will release the algorithm for free if he dies. Tankado is found dead in Seville, Spain. Fletcher, along with her fiancé, David Becker, a skilled linguist with eidetic memory, must find a solution to stop the spread of the code.