In the past few years, aging police officer John McClane (Bruce Willis) has spent his time in and out of Alcoholics Anonymous and is divorced from his wife. He is no longer employed by the police department, but has found a position with the United States Department of Homeland Security chasing dangerous computer hackers. On his way to turn a young hacker named Matt Foster (Justin Long) into custody, he experiences a modern version of a terrorist attack first hand when he gets stuck in traffic. A group of hackers, headed by Greg Pope (Timothy Olyphant), have struck at the vulnerable United States computer infrastructure and are systematically shutting it down, beginning with the traffic light system.

As they move on to crash banking networks and the stock market, the indices plummet and America's economy becomes crippled. As usual, McClane gets caught up in the middle of the excitement. It turns out that the recently captured Matt seems bright enough to understand exactly what the terrorists are doing and what steps they are taking to achieve their mission. The sophisticated criminals have a nearly foolproof plan to bring the country to a halt, but they didn't count on an old-school cop and his computer-smart buddy to figure things out. Soon, the crime-fighting duo of McClane and Foster are pulling out all the stops and matching wits with the terrorists, hoping to figure out the motives behind their actions and to stop them before the entire nation collapses.