Digital Fortress by Dan Brown is a novel of deception and betrayal in the nation's most secret and powerful government organization, the National Security Agency (the NSA). Commander Trevor Strathmore believes that a former NSA employee has successfully created an unbreakable algorithm, which threatens the security of the nation and the future of the agency itself.

An ambitious man, Strathmore recognizes the opportunity to turn the situation around to his own benefit. When Strathmore realizes he needs assistance, he calls in the NSA's head cryptographer, Susan Fletcher.

In a race against time, Susan uncovers the astounding truth about the NSA's deputy director as the nation's most classified information is in grave danger of being exposed to the world and the life of the man she loves hangs in the balance.

Susan's fiancé, David Becker phones to tell her that he has been called out of town unexpectedly and their weekend plans will have to be delayed. As Susan waits to hear from David, she receives another call from her boss, Commander Trevor Strathmore, Deputy Director of the NSA. He informs Susan there is an emergency at work and she must report to the Cryptography facility immediately.

When Susan arrives, she finds that Commander Strathmore is the only one working on the Cryptography floor. He tells Susan that a former NSA employee, Ensei Tankado, has designed a program called Digital Fortress that he claims can create unbreakable codes. Susan is skeptical until Strathmore informs her that the NSA's top-secret, multi-billion dollar decryption computer, TRANSLTR, has been working for sixteen hours and is unable to break the code.

Tankado was employed with the NSA on a work visa from Japan. He helped to build TRANSLTR but was outraged when he discovered that the NSA would be able to use the computer to read the emails of private citizens. Tankado saw the NSA's ability to read private emails as a violation of human rights. He quit his job and threatened to go public about the NSA's top-secret computer. Tankado's capture and deportation was widely publicized and his reputation was ruined.

Strathmore tells Susan that the previous month Tankado had called to warn him that he was perfecting an algorithm that could create unbreakable codes. Tankado told Strathmore he would destroy Digital Fortress if the NSA would go public about TRANSLTR. Strathmore did not believe him, so Tankado moved forward by placing a complimentary copy of the program on his website. Every software company in Japan downloaded Digital Fortress but no one can open it without Tankado's pass code, which is being auctioned off to the highest bidder.

To ensure his safety, Tankado left Japan and gave a copy of his pass code to an anonymous third party known only as North Dakota. If anything should happen to Tankado, North Dakota is to publish the pass code, making it free to everyone. Strathmore informs Susan that he had just received word that morning that Tankado was found dead in Seville, Spain.

In a desperate attempt to discover Tankado's secret partner before the word gets out that Tankado is dead, Strathmore asks Susan to run a tracer on an email account for 'ndakota' that has been set up on an anonymous server.

Susan finds out that Strathmore sent David to Spain. Strathmore believes that Tankado would have had the pass code on him at the time of his death and sent David to retrieve all of Tankado's personal belongings. Susan is enraged that the deputy director would send her fiancé, a private citizen, instead of contacting a government official to do the job. Strathmore tells Susan that he cannot afford to take the risk of anyone finding out about Tankado's death and he sent David, because he needed a private courier that he could trust.

When David arrives in Spain to retrieve Tankado's effects, he has no idea what he is supposed to bring back. He was only given strict orders to leave nothing behind. While at the morgue, David notices that a ring had been removed from Tankado's finger. When he asks the Spanish officer if Tankado was wearing any jewelry, the officer informs him that a Canadian tourist mentioned something about a ring but the officer did not believe the man's story was true.

David calls Strathmore and tells him about the ring. Strathmore orders David to find the ring, telling him it is a matter of national security. David finds the Canadian tourist, who tells him a German and his female escort took the ring. David locates the German and his escort. The woman tells David that she gave the ring to a young teenage girl in the park. David talks to a boy who knows the girl and he tells David the girl was trying to hock the ring to buy a plane ticket home to Connecticut.

David goes to the airport and retrieves the ring from the girl. He later finds her dead and discovers he has an assassin following him. The murderer chases David through the narrow streets of the small village of Santa Cruz and tries to kill him in the church.

David escapes to the bell tower and the killer follows him, thinking he has him trapped. David is able to outwit the assassin, who stumbles down the stairs to his death. David is shot with a stun gun by two government agents who put him in the back of their van along with the body of the assassin.

As David is tracking down the ring in Spain and Susan is running a tracer on the email account that Strathmore had given her, a computer software entrepreneur in Tokyo receives a phone call from an American claiming to be Tankado's partner, North Dakota. The caller tells the business owner, Tokugen Numataka, that Tankado is dead and he can provide him with both pass codes for the same amount of money that Numataka has bid on Tankado's pass code. The caller tells Numataka that he will be in touch. Numataka grows impatient and has his switchboard operator trace the call, which turns out to be Strathmore's personal cell phone.

Greg Hale, a fellow cryptographer that Susan dislikes, comes in while she is tracing the email. When Susan discovers emails from Tankado to North Dakota on Hale's computer, she suspects that he is Tankado's secret partner.

Meanwhile, Phil Chartrukian, an NSA technician discovers that TRANSLTR has a virus. Strathmore tells the technician that he and Susan are running a diagnostic on the computer and orders the technician to leave the premises. Chartrukian defies the commander's orders and goes to the sublevels to shut off TRANSLTR's power. A struggle ensues between Strathmore and Chartrukian and the technician falls to his death. Chartrukian's body shorts out the generator and the main power is lost on the Cryptography floor.

Hale witnesses Chartrukian's death but, when he tries to tell Susan that Strathmore is the one who killed the technician, she does not believe him. Susan thinks Hale is the killer and is afraid for her life. She tells Strathmore that Hale is North Dakota and begs him to let her leave but Strathmore refuses to let her go.

Strathmore confesses to Susan that he has not been entirely honest with her. He explains that he is trying to open Digital Fortress so he can modify the program. He wants to make it possible, without the public's knowledge, for the NSA to access Digital Fortress with a pass code. The public will use Digital Fortress thinking that their messages cannot be intercepted. They will not realize that the NSA will have complete and total access to all their communications.

Hale knows Strathmore's plans. He takes Susan hostage in an attempt to get both of them away from Strathmore and safely out of the building. Susan believes that Hale is going to hurt her. Strathmore knocks Hale unconscious, then ties him up. Believing that Hale is North Dakota, Strathmore tells Susan she needs to search Hale's computer for the pass code.

TRANSLTR begins to overheat and warning sirens begin to blare. Strathmore leaves the room to abort TRANSLTR. Susan is still searching Hale's computer when he regains consciousness. Hale proclaims his innocence but Susan still does not believe him. Then she discovers that North Dakota is a fake alias. Tankado was sending email messages to himself the entire time. There is no third party and Digital Fortress is a ruse.

At the same time Susan makes this discovery, Strathmore realizes that he is unable to abort, because he had bypassed TRANSLTR's filters and allowed a virus into the system. The virus was now heading for the main databank.

Susan rushes to the sublevels to shut off all power to TRANSLTR. Strathmore goes to his computer and begins typing a suicide letter. As Susan is heading down the ladder, she hears a gunshot. She races back upstairs and finds Hale with a gunshot wound to the head. The gun is lying beside him along with a suicide note.

Strathmore rushes in after Susan discovers the body. He drapes his jacket over Susan's shoulders and has her sit down while he goes to shut off the power. While he is gone, a pager goes off in Strathmore's pocket. Susan reads the message and finds that David, along with several other victims, has been murdered. She realizes that the page is coming from an assassin that Strathmore has hired.

When Susan confronts Strathmore, he begs her to understand. As she tries to get away from him, TRANSLTR explodes and Strathmore is engulfed in flames. Susan escapes and finds herself in the underground corridors of the NSA. She is in a state of shock and believes her fiancé is dead.

The Director of the NSA and his personal assistant bring Susan into the underground chamber where the main databank is located. The virus that Strathmore let into the system is actually a worm that will destroy the main databank's filters, allowing public access to classified government information.

Everyone in the room is trying to figure out the kill-code that will stop the worm. Susan tells them that the pass code is engraved on a ring that belonged to Tankado, who was killed in Spain.

The Director orders a live video be streamed into the room. Two agents in a van begin to brief the room on the events that have been taking place in Spain. When Susan sees David and realizes he is alive, she is ecstatic. David hears her voice and regains consciousness.

David tells everyone in the room that he has the ring. He reads the inscription but it turns out to not be the pass code. Susan discovers a code in Tankado's program that turns out to be a puzzle. Seconds before the main databank's filters are destroyed, the puzzle is solved and the databank is saved.

In the epilogue, Numataka discovers that Tankado was the son he had abandoned at childbirth.