Day 4 begins with the bombing of a commuter train which (unbeknownst to CTU at the time) enables the theft of a device known as the Dobsen-type Override which could be used to take control of (and meltdown) United States nuclear power plants. With Jack Bauer working for Secretary of Defense James Heller, his job brings him to CTU as a visitor on this very day.
Believing that something terrible is about to happen, Jack begs CTU director Erin Driscoll to reinstate him and begins to interrogate a suspect. The suspect reveals that James Heller and Audrey Raines are the primary targets just as they are kidnapped. When Jack saves a programmer who discovered evidence of a cyber-attack in progress (a friend of Chloe O'Brian), he allows one of the terrorists to escape and lead him to where Raines and Heller are being held. Even though Jack rescues them, the mass streaming of the planned execution video gives the terrorists the opportunity they need to launch a denial-of-service attack and begin using the override.