Season four begins 18 months later. CTU is now headed by Erin Driscoll, a steely government agent who made firing Jack one of her first priorities upon taking over. After the explosion of a commuter train, Jack, who is now working for Secretary of Defense James Heller, while secretly romantically involved with Heller's married daughter Audrey, suddenly finds himself heading back to CTU for a meeting with Driscoll on Heller's behalf.