The FBI responds to a brief computer outage at their Cyber-Security Division by tracing down top computer hackers, finding several of them have been killed. Taking others into protective custody, the FBI asks NYPD detective John McClane (Bruce Willis) to collect Matthew "Matt" Farrell (Justin Long). McClane arrives in time to prevent Farrell from being killed by an assassin, Mai Linh (Maggie Q), working for Thomas Gabriel (Timothy Olyphant). On route to Washington DC, Farrell explains he had written a portion of security code for a large sum of money from Mai.
As they arrive in DC, Gabriel orders his own crew of hackers to take control of the transportation grids and stock market, while nationally broadcasting a message threatening the United States. Farrell recognizes this as the start of a "fire sale", an attack designed to target the nation's reliance on computer controls, such that "everything must go". McClane and Farrell are taken by police escorts to the secured FBI headquarters, but Mai, using the hijacked grids, reroutes the convoy into the path of an assault helicopter. McClane is able to take down the helicopter by launching a police car over a damaged toll booth into it. As McClane and Farrell recover, Gabriel initiates a second broadcast, showing a simulated explosion of the United States Capitol building, sending the public into panic. Farrell recognizes that the next target of the "fire sale" is likely the power grid, and the two drive to a utility superstation in West Virginia. They find a team led by Mai breaking into the station's controls. McClane is able to kill the entire team, including Mai, and obtain video footage of Gabriel, which they relay to FBI headquarters. Enraged over Mai's death, Gabriel uses remote control of the station to redirect the natural gas supply into it. McClane and Farrell escape before the station explodes, leaving much of the eastern seaboard without power.
Farrell directs McClane to a fellow hacker, Frederick "Warlock" Kaludis (Kevin Smith), in Baltimore. Warlock, running his computer systems from several generators, identifies the piece of code Farrell wrote as a means to access data at a Social Security Administration building at Woodlawn, Maryland. They realize this building is a front for a National Security Agency government facility, designed by Gabriel to download all personal and financial records in case of a cyber-security emergency such as this one generated by Gabriel himself. Warlock is also able to explain Gabriel's motivation: a talented hacker, he was once a top expert for the Department of Defense. However, Gabriel was fired and his reputation was tarnished when he tried to sound the alarm about America's vulnerability to cyber-warfare. Gabriel detects Warlock's hack and speaks with him, Farrell, and McClane, and reveals that he has kidnapped McClane's estranged daughter, Lucy (Mary Elizabeth Winstead).
McClane and Farrell race to the Woodlawn facility. As McClane deals with Gabriel's men, Farrell discovers that Gabriel is downloading all the information onto a portable computer system, and he is able to encrypt the data after it is completed. Gabriel is forced to take Farrell as they escape the facility. McClane follows, hijacking Gabriel's semi and gains Warlock's help to trace Gabriel's own vehicle. With McClane in pursuit, Gabriel hacks into the military's computers to deceive a United States Marine Corps F-35B Lightning II pilot into believing that McClane is a terrorist. McClane is able to escape the assault when pieces from the damaged truck fall into the jet intake of the aircraft from a highway exit ramp. McClane makes his way to a nearby warehouse to which Warlock tracked Gabriel. Though McClane dispatches most of Gabriel's men, Gabriel's last henchman, Emerson, wounds him in the shoulder. When Gabriel holds McClane from behind, McClane manages to make Gabriel fire a gun through McClane's wounded shoulder, killing Gabriel. Farrell then kills Emerson with a gun that had fallen to the floor earlier in the confrontation. As the FBI arrives to tend to McClane and Farrell's wounds and clean up, McClane is displeased to see Lucy and Farrell taking a romantic interest in each other.
