Set 18 months after Day 3, the thriller's fourth season finds Jack Bauer (Kiefer Sutherland) now employed by Defense Secretary Heller (William Devane) and striving to stop Middle Eastern terrorist Habib Marwan (Arnold Vosloo), who unleashes a large-scale attack on the U.S. that begins with the kidnapping of Heller and Heller's daughter Audrey (Kim Raver), who's also Bauer's girlfriend.

Other aspects of Marwan's plot: forcing all of the nation's nuclear plants into meltdown; stealing a stealthfighter to bring down Air Force One; and detonating a nuclear device. As the day winds down, Bauer and the government also have to deal with the Chinese, who want someone to answer for a raid on one of their consulates.