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 orders 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”. [source]