NYPD Detective John McClane has faced some brutal terrorists in his career, but now, he's about to face what may be his most dangerous enemy yet. John, who occasionally does work for the U.S. Department of Homeland Security, receives a call to bring in a hacker named Matt Farrell, because there has been a breach in an FBI computer system. Farrell is one of the many hackers who are suspected in the attack. After John gets to Matt's apartment, men show up and try to kill John and Matt and they barely escape with their lives. A group of terrorists led by Thomas Gabriel are systematically shutting down the U.S. computer infrastructure. Luckily, Matt knows enough about hacking to figure out how Gabriel is shutting down the computers. Gabriel plans to bring the country to its knees, but doesn't count on John and Matt. Gabriel wants them out of the way. An enraged Gabriel kidnaps John's daughter, Lucy. Gabriel may think he now has the upper hand on John, but he has no idea how far John is willing to go to rescue Lucy.