Story
He was once part of the Volksfrei West German terrorist group but was expelled from the group, presumably for being too radical. At the time the film takes place, Gruber leads a group of heavily armed, well-funded mercenaries and organizes the takeover of a massive Los Angeles skyscraper that houses the offices of the Nakatomi Corporation. During this, they take over 30 employees hostage. When the FBI tries to negotiate, Gruber orders the release of various terrorist leaders around the world and a couple of helicopters on the building's roof. However, this is all an elaborate ruse to distract the police from the heist of $640 million in untraceable and negotiable bearer bonds that are kept in the building's vault.
Gruber takes advantage of the company's Christmas Eve party when only a limited number of people are in the building. His men take out the limited group of security guards, seal off the building, and cut the phone lines. He then leads his men to the party upstairs and takes all the guests hostage except for NYPD cop John McClane, in town visiting his estranged wife Holly. John managed to make it to a stairwell and head upstairs into the unfinished portion of the building. Gruber talks to the head of international trading, Joseph Takagi, and tries to coax the building's vault code out of him, but Takagi claims that he does not know the code and bravely asserts that they will just have to kill him. Gruber obliges this request and shoots Takagi in the head. John sees this and flees, realizing that the terrorists are hostile. In an attempt to get help, John pulls the fire alarm. Gruber tracks what floor this came off of and sends a terrorist onto the floor to kill him.
When John kills the terrorist, Gruber simply sends more men after him and proceeds with his master plan, which involved waiting for the FBI to cut the power, which would open the final magnetic lock on the vault. The hostages would then be sent up to the helipad on the roof, which would be blown up with C-4, confusing the feds and allowing the villains to escape in an ambulance. Although the charges had been put in place, John killed a henchman and took the detonators he had been carrying. Meanwhile, McClane is initially able to interfere minimally such as finally getting the local authorities to respond sooner than Gruber would have preferred, although they were intended to play an unwitting part of his plan as well.
A cat-and-mouse game follows between John and Gruber, in which the two communicate through the use of walkie-talkies where they learn more about each other. Gruber found out who John was through the assistance of a Nakatomi executive who met the cop before the attack, but he did not find out John's relation to Holly, who was using her maiden name, Gennero. In an act of desperation, Gruber headed up to the unfinished floors to retrieve the detonators, where he ended up meeting John. Using an alias and an improvised American accent, he posed as an escaped hostage, but he gave himself away by holding his cigarette in European fashion. His henchmen arrive, and a violent shootout ensues. Hans orders his men to shoot up the glass walls of the offices. Despite being barefoot, John runs over broken glass and escapes. However, Gruber retrieves the detonators.
