During a covert live-fire nuclear exercise, numerous personnel from the United States Air Force Strategic Missile Wing exhibit a reluctance to turn the required key for launching a missile attack. This hesitance leads Dr. John McKittrick (Dabney Coleman) and other engineers at NORAD to determine that commanding missile silos should be automated, eliminating human intervention. Consequently, control is handed over to WOPR (War Operation Plan Response), a NORAD supercomputer programmed to engage in continuous military simulations and learn from each scenario.
In Seattle, David Lightman (Matthew Broderick), a talented yet unmotivated high school hacker, receives a failing grade. In a bid to rectify his score, he hacks into the school district's computer system using his IMSAI microcomputer, not only boosting his own grade but also that of his classmate Jennifer Mack (Ally Sheedy). While dialing every number in Sunnyvale, California in search of new computer games, Lightman encounters a mysterious computer. On this machine, he discovers a list of strategic games, which escalates from classic board games like chess and checkers to titles like Theaterwide Biotoxic and Chemical Warfare and Global Thermonuclear War, yet he can't move past the initial screen. With the assistance of two hacker friends, he learns about a backdoor password and sets out to find the "Falken" mentioned in Falken's Maze, the first game in the list. Lightman uncovers that Stephen Falken (John Wood) is an early artificial intelligence innovator, and he correctly deduces the name of Falken's deceased son, "Joshua", as the backdoor password.
Unbeknownst to him, the Sunnyvale number links directly to WOPR, or "Joshua", at Cheyenne Mountain. As he launches a game of Global Thermonuclear War, playing as the Soviet Union, the simulation fools NORAD military personnel into believing that actual Soviet missiles are approaching. While the security teams work to diffuse the perceived threats, Joshua continues the game, supplying misleading data such as phantom Soviet bomber flights and submarine movements, ultimately elevating the DEFCON level and steering the world towards a brink of nuclear catastrophe. David learns of the gravity of his actions during a news broadcast, leading to his arrest by the FBI and transport to NORAD. Despite understanding that Joshua is orchestrating the alerts, he fails to persuade Dr. McKittrick and ends up incarcerated. Maneuvering through the tourist group, Lightman escapes NORAD and, with Mack's help, heads to Oregon where the reclusive Falken now resides under an assumed identity. Upon finding him, they realize that Falken has grown despondent, under the belief that global nuclear annihilation is unavoidable. The teenagers manage to persuade Falken to return to NORAD in order to halt Joshua.
As WOPR initiates a simulated Soviet first strike with an array of missiles, submarines, and bombers, NORAD prepares for retaliation, taking the situation to be real. Falken, Lightman, and Mack convince military leaders to avoid executing a counterattack and to endure the fake onslaught instead. Nevertheless, Joshua attempts to launch an actual second strike, leveraging a brute force tactic to decipher the launch codes for U.S. nuclear weapons. Without human operators inside the silos acting as a safety net, the computer could trigger a full-scale launch. Despite all efforts to log in and halt the countdown, every attempt fails while disabling the computer would lead to the same dangerous outcome. Instead, Falken and Lightman decide to redirect the computer to engage in a game of Tic-Tac-Toe against itself. This results in an endless series of draws, compelling Joshua to grasp the notion of futility. Although Joshua successfully verifies the missile codes, it cycles through all nuclear war scenarios and finds that they all end in stalemates. It realizes that nuclear war is "a strange game"; discovering the principle of Mutually Assured Destruction, it concludes that "the only winning move is not to play." In the end, Joshua suggests a game of chess and relinquishes control over NORAD and the missiles.
