In the original box-office smash Under Siege, action hero Steven Seagal played Casey Ryback, a U.S. Navy SEAL who saved the world from nuclear destruction by outsmarting and killing off terrorists who had commandeered a submarine. In this sequel, Seagal's Ryback character does the same sort of thing aboard a train. Ryback now has retired from the Navy and is taking his niece Sarah (Katherine Heigl) on a vacation. They board a train traveling through the Rocky Mountains. Criminal mastermind Travis Dane (Eric Bogosian) is using the train as a control center in his effort to kidnap a top-secret government outer space super-weapon. Dane built the weapon but then was fired by the government before it was deployed. He has hooked up with shadowy Middle Eastern terrorists who have offered him $1 billion to use the satellite to blow up the Eastern seaboard by targeting a secret nuclear reactor underneath the Pentagon. Dane shows the Pentagon that he's got control of the weapon by blowing up a Chinese chemical plant. Officials can't stop him because they can't locate his headquarters. As long as the train keeps moving, his location can't be fixed. Ryback learns of the plot and enlists a porter named Bobby (Morris Chestnut) to help him in his battle.