During a surprise nuclear attack drill, many United States Air Force Strategic Missile Wing controllers prove unwilling to turn the keys required to launch a missile strike. Such refusals convince John McKittrick and other North American Aerospace Defense Command (NORAD) systems engineers that missile launch control centers must be automated, without human intervention. Control is given to a NORAD supercomputer known as WOPR (War Operation Plan Response, pronounced "whopper"), or Joshua, programmed to continuously run war simulations and learn over time.
David Lightman, a bright but unmotivated Seattle high school student and hacker, uses his IMSAI 8080 computer and modem to access the school district's computer system and change the grades for himself and his friend and classmate, Jennifer Mack. Later, while war dialing numbers in Sunnyvale, California, to find a computer game company, he connects with a system that does not identify itself. Asking for games, he finds a list including chess, checkers, backgammon and poker, along with titles such as "Theaterwide Biotoxic and Chemical Warfare" and "Global Thermonuclear War", but cannot proceed further. Two hacker friends explain the concept of a backdoor password and suggest tracking down the Falken referenced in "Falken's Maze", the first game listed. David discovers that Stephen Falken was an early artificial-intelligence researcher, and guesses correctly that the name of Falken's deceased son (Joshua) is the password.
Unaware that the Sunnyvale phone number connects to WOPR at the Cheyenne Mountain Complex, David initiates a game of Global Thermonuclear War, playing as the Soviet Union while targeting American cities. The computer starts a simulation that briefly convinces NORAD military personnel that actual Soviet nuclear missiles are inbound. While they defuse the situation, WOPR nonetheless continues the simulation to trigger the scenario and win the game, as it does not understand the difference between reality and simulation. It continuously feeds false data such as Soviet bomber incursions and submarine deployments to NORAD, pushing them to increase the DEFCON level toward a retaliation that will start World War III.
David learns the true nature of his actions from a news broadcast, and FBI special agents arrest him and take him to NORAD. He realizes that WOPR is behind the NORAD alerts, but he fails to convince McKittrick (who believes David is working for the Soviets) and is charged with espionage. David escapes NORAD by joining a tourist group and, with Jennifer's help, travels to the Oregon island where Falken lives under the alias "Robert Hume". David and Jennifer find that Falken has become despondent, believing that nuclear war is inevitable and as futile as a game of tic-tac-toe between two experienced players. The teenagers convince Falken that he should return to NORAD to stop WOPR.
WOPR stages a massive Soviet first strike with hundreds of missiles, submarines, and bombers. Believing the attack to be genuine, NORAD prepares to retaliate. Falken, David, and Jennifer convince military officials to delay the second strike and ride out the supposed attack until actual weapons impacts are confirmed. When the targeted American bases report back unharmed, NORAD prepares to cancel the retaliatory second strike. WOPR tries to launch the missiles on its own using a brute-force attack to obtain the launch codes. Without humans in the control centers as a safeguard using the two-man rule, the computer will trigger a mass launch. All attempts to log in and order WOPR to cancel the countdown fail. Disconnecting the computer is discussed and dismissed, as a fail-deadly mechanism will launch all weapons if the computer is disabled.
Falken and David direct the computer to play tic-tac-toe against itself. This results in a long string of draws, forcing the computer to learn the concept of futility and no-win scenarios. WOPR obtains the launch codes, but before launching, it cycles through all the nuclear war scenarios it has devised, finding that they all result in draws as well. Having discovered the concept of mutual assured destruction ("WINNER: NONE"), the computer tells Falken it has concluded that nuclear war is "a strange game" in which "the only winning move is not to play." WOPR relinquishes control of NORAD and the missiles and offers to play "a nice game of chess".
