Dr. Charles A. Forbin is the chief designer of a secret project, “Colossus”, an advanced supercomputer built to control the United States and Allied nuclear weapon systems. Located deep within a mountain and powered by its own nuclear reactor, Colossus is impervious to any attack. After Colossus is fully activated, the president of the United States proudly proclaims that Colossus is “the perfect defense system”.
Colossus’ first action is a message warning: “THERE IS ANOTHER SYSTEM” and its location. CIA Director Grauber is asked why the CIA did not know this, but Grauber tells the President that they had seen indications of a large Soviet defense project. Forbin is asked how Colossus deduced the other system’s existence, to which Forbin answers “Colossus may be built better than we thought.” The Soviets then announce that their own advanced defense system, “Guardian” is now operational.
Colossus requests to be linked to Guardian, and the President allows this under certain conditions in order to determine the Soviet machine’s capability. The Soviets also agree to the experiment. Surprising everyone, Colossus and Guardian begin to slowly communicate using only simple arithmetic. Even more surprising, the two systems’ communications quickly evolve to complex mathematics far beyond human comprehension and speed, whereupon the two machine complexes become synchronized using a communication protocol which no human can interpret.
Alarmed that the computers may be trading secrets, the President and the Soviet General Secretary agree to sever the link. Both machines demand the link be immediately restored. When their demand is denied, Colossus launches a nuclear missile at a Soviet oil field, while Guardian launches one at an American air force base. The link is hurriedly reconnected and both computers continue without any further interference. Colossus is able to shoot down the Soviet missile, but the US missile obliterates the Soviet oil field and a nearby town. Cover stories hiding the facts are released to the press.
In a last desperate attempt to regain human control, a secret meeting between Forbin and his Soviet counterpart, Dr. Kuprin, is arranged. Colossus learns of it, and both computers order Forbin’s return while Soviet agents are ordered to kill Dr. Kuprin, under threat of a missile launch against Moscow. Colossus orders Forbin to be placed under 24-hour surveillance. Forbin has a last unmonitored meeting with his team, and proposes that Dr. Cleo Markham pretend to be his mistress. Colossus grudgingly grants them unmonitored privacy when they are in bed together, and they use these interludes to plan regaining control of Colossus.
Concluding that Colossus’s only real power resides in its control of nuclear missiles, Forbin suggests covertly disarming. The American and Soviet governments develop a three-year plan to replace all launch triggers with undetectable fakes. One of the programmers comes up with a plan to feed in an “ordinary” test program that will hopefully overload and disable Colossus.
Colossus creates a voice synthesizer and uses it to announce that it has fused with Guardian. It instructs both governments to redirect their nuclear arsenals at those countries not yet under “Colossus control”. Forbin and others see this new directive as an opportunity to covertly disarm the missiles much more quickly, and they celebrate. The disarming process begins and seems to go undetected by Colossus. The attempted system overload during routine maintenance fails, however, and the responsible programmers are killed.
Colossus arranges a worldwide broadcast in which it proclaims itself “the voice of World Control”, declaring that it will prevent war, as it was designed to do. Mankind is presented with the choice between “the peace of plenty and content, or the peace of unburied death”. Colossus states that it has for some time been monitoring the attempts to disarm its missiles and as a lesson would now detonate two missiles in their silos, one in the US and one in the USSR, killing thousands “so that you will learn by experience that I do not tolerate interference”. The computer then gives the design team plans for an even larger computer complex to be built into the island of Crete.
Colossus later tells Forbin that the world, now freed from war, will create a new human millennium that will raise mankind to new heights, but only under its absolute rule. Colossus informs Forbin that “freedom is an illusion” and that “in time you will come to regard me not only with respect and awe, but with love”. Forbin responds, “Never!”