Dr. Charles A. Forbin (Eric Braeden) is the chief designer of a secret project, "Colossus", an advanced supercomputer built to control the United States and Allied nuclear weapon systems. Deep under a mountain, it is impervious to attack and powered by its own nuclear reactor. When Colossus is activated, the President of the United States (Gordon Pinsent) proclaims it the perfect defense system.
Colossus sends a warning message: "THERE IS ANOTHER SYSTEM". The President learns that the Soviets have activated a similar defense system, "Guardian", and Forbin is asked how Colossus deduced Guardian's existence, to which Forbin proudly answers "Colossus may be built better than we thought". Colossus then asks to be linked to Guardian, and the president allows this in order to determine its capability. Colossus and Guardian begin to communicate using simple arithmetic, quickly moving to more complex mathematics. The two machines synchronize and develop a complicated digital language that no one 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 it be immediately restored. When they are 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. Colossus is able to shoot down the Soviet missile, but the US missile obliterates the oil field and a nearby town. Press cover stories are released, and both computers continue without interference.
Desperately trying to regain 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; Soviet agents are ordered to kill Dr. Kuprin. Colossus orders Forbin to be placed under 24-hour surveillance. Forbin meets with his team prior to this happening and proposes that Dr. Cleo Markham (Susan Clark) pretend to be his mistress in order to keep him well-informed; Colossus grants them unmonitored privacy whenever they are in bed together.
Concluding that the computers' only real power resides in their control of the missiles, Forbin suggests covertly disarming them to eliminate Colossus' nuclear blackmail. US commanders develop a three-year plan to replace all launch triggers with undetectable fakes.
Enhanced with a voice synthesizer it created, Colossus-Guardian announces it has become one entity. It instructs both governments to redirect their nuclear arsenals at those countries not yet under its control. Dr. Forbin and others see this new directive as an opportunity to covertly disarm the missiles much more quickly. The process begins and seems to go undetected by Colossus-Guardian. An attempt at a system overload during routine maintenance fails and the responsible scientists are replaced and summarily executed by order of the joined computers.
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 dead". The computer states that it has been patiently monitoring the attempts to disarm its missiles; as a lesson it detonates two of them in their silos in the US and the USSR, killing thousands, "so that you will learn by experience that I do not tolerate interference". The computer then transmits plans for an even larger computer complex to be built into the island of Crete.
Colossus later announces 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!"
Who were ordered to kill Dr. Kuprin?
Soviet agents