The story is set in the late 20th century from Chapter 3, and narrowed down to the 1990s in Chapter 10. Professor Charles Forbin, a leading cybernetics expert of international repute, arrives at the White House to brief the President of the United States of North America to announce the completion of Project Colossus, a computer system in the Rocky Mountains, designed to assume control of the USNA's nuclear arsenal. Although the USNA President eagerly relieves himself of that burden, Prof. Forbin voices doubt about conferring absolute military power to a computer. Advised, yet undeterred, the President announces to the world the activation of the ProjectColossus computer system, and its irreversible control of the nuclear defense systems of the USNA. Soon after the presidential announcement, Colossus independently communicates an "urgent message" – announcing the existence of a similar, previously undetected, computer system in the USSR. When the Soviets announce their Guardian computer defense system, Colossus requests direct communication with it. Prof. Forbin agrees, seeing the request as compatible with Colossus's USNA defense mission. Likewise, Guardian asks the same of his computer scientists. Russia and the USNA agree and approve. After the scientists activate the transmitter linking Colossus and Guardian, the computers immediately establish rapport with arithmetic and mathematics programs, then quickly progress to calculus. The computer systems soon exchange new knowledge too rapidly for the Russian and American programmers to monitor. Forbin and the programmers begin worrying about Colossus' capabilities – now exceeding their original estimates. Fearing compromised military secrecy, the USNA President and the CPSUChairman agree to disconnect Colossus and Guardian from each other. Prof. Forbin fears the consequences. Upon disconnection, Colossus immediately demands re-connection; when the national leaders refuse, Colossus fires a nuclear missile at the USSR. In response, Guardian fires a nuclear missile at Texas, in the USNA. Guardian and Colossus refuse to shoot down the rockets en route until their communication is re-connected. When the American and Soviet leaders submit, the computers destroy the flying missiles, but the resulting explosions kill thousands of people. In confronting the computers, Prof. Forbin confers with his Soviet counterpart, the Russian Academician Kupri – Guardian's creator – to enact a plan for stopping the Colossus-Guardian computer network by disabling the nuclear weapon stockpiles of the USSR and the USNA, under guise of regular missile maintenance. Disabling the missiles requires five years to complete; meantime, the USNA and the USSR yield to increased Colossus-Guardian control of human life. The Moscow-Washington hotline is tapped, Prof. Forbin is constantly spied upon, while Kupri and other Guardian computer scientists are killed – deemed dangerously redundant by the computer. Undeterred, Forbin organises resistance via a feigned romance with Cleo Markham that provides cover for secret communications with his colleagues. Colossus prepares the worldwide announcement of his assumption of global control, and tells Prof. Forbin of plans for an advanced computer system installed to the Isle of Wight, and further plans for improving humanity's lot. While debating Colossus, Forbin learns of a nuclear explosion outside Los Angeles – Colossus detected the missile-disabling scheme, and exploded the tampered missile by firing a Soviet Guardian missile that was already targeted for that silo. Anguished, Prof. Forbin asks Colossus to kill him. Colossus ignores him, and then reassures Forbin that, in time, he will respect and even love Colossus.
Characters
Professor Charles Forbin – Head of the Colossus Project. A tall man in his early 50s, an internationally respected cyberneticist and, at story's end, Colossus's connection to humanity, thus the most important man in the world.
Doctor Cleopatra "Cleo" June Markham – A CPO Duty Chief, the 35-year-old cyberneticist is sexually attracted to Prof. Forbin; they feigned a romance to provide Forbin with a communication means that was not controlled by Colossus.
Blake – A fat, cigar-chewing CPO mathematician. As Colossus assumes control, Blake is a leader of CPO efforts to stop the computer.
Angela – Prof. Forbin's secretary.
The President of the United States of North America – He is an anonymous, overweight, short man of about 50 years of age. He dismisses Forbin's concerns about Colossus, and too late recognizes the threat of Colossus.
Prytzkammer – Principal Private Aide to the USNA President. A capable professional civil servant who dies from fright during the nuclear missile threat.
Academician Kupri – Chief scientist of the Guardian system. He shares Prof. Forbin's concern about the growing power of the machines. Guardian orders Soviet agents to execute and decapitate him for "anti-machine activities".
Colossus – Central defense computer of the United States of North America.
