In season three of 24, Jack Bauer (Kiefer Sutherland), who was in charge of a special field operations unit of the Counter Terrorist Unit, fought to stop a viral terrorism threat before it could kill millions of people.
Season four begins 18 months later. CTU is now headed up by Erin Driscoll (Alberta Watson), 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 (William Devane) and is also romantically involved with Heller’s married daughter Audrey Raines (Kim Raver), suddenly finds himself heading back to CTU for a meeting with Driscoll.