Forbin is the designer of an incredibly sophisticated computer that will run all of America's nuclear defenses. Shortly after being turned on, it detects the existence of Guardian, the Soviet counterpart, previously unknown to US Planners. Both computers insist that they be linked, and after taking safeguards to preserve confidential material, each side agrees to allow it. As soon as the link is established the two become a new Super computer and threaten the world with the immediate launch of nuclear weapons if they are detached. Colossus begins to give it's plans for the management of the world under it's guidance. Forbin and the other scientists form a technological resistance to Colossus which must operate underground.

Charles Forbin, director of a U. S. defense project, announces to the world that Colossus, a computer programmed to defend the Western world, is finally operational; the self-contained system, located beneath the Rocky Mountains, will provide the ultimate nuclear deterrent to war. The computer is in operation for only a short time before it detects the presence of Guardian, a similar system located in the Soviet Union, and Colossus requests to be put in contact with his counterpart. The unwary scientists comply, and soon the two systems are communicating in computer language. American cold war strategists, fearful that secret information will be disclosed, pressure the President into severing the line of communication. The connection is then terminated by joint United States-Russian consent, but Colossus, now assuming a personality of its own, threatens retaliation unless the line is restored. Colossus and Guardian prepare to launch a missile attack upon each other's countries. At the last minute, the Americans and Russians restore communication, but not before one of the U. S. armed missiles destroys a Russian town. Forbin and his Russian counterpart, Dr. Kuprin, make plans to destroy the computers, but the plot is detected, and the computers, now working together, order Kuprin's execution. Forbin, placed under close surveillance by Colossus, continues to plan the overthrow of the computers; with his attractive chief aide, Cleo Markham, posing as his mistress, Forbin receives permission from Colossus to spend several nights alone with her. Together they attempt to overload the computer, but it learns of their plans and orders the execution of two technicians aiding in the plot. After blowing up missile sites in both the United States and Russia in retaliation for the sabotage attempt, Colossus tries to convince Forbin that they can coexist peacefully, as long as the computer is in control.