Matthew Broderick stars as David Lightman, a young hacker who accidentally logs on to the Department of Defense's network. Thinking that he's found a cool new computer game manufacturer, David plays checkers, chess, and other more intriguing games like Global Thermonuclear War. Realizing that their system has been tampered with, military operatives arrest him. However, the computer continues to play the "game" of thermonuclear warfare without David and generates the very real threat of World War III. In an attempt to prevent global disaster, David and his girlfriend, Jennifer (Ally Sheedy), search desperately for the scientist who designed the system before the goverment computer initates a full-scale nuclear war.