Eighteen months have elapsed since the conclusion of Day 3. A commuter train bound for Los Angeles explodes. A man steals a suitcase from a passenger in the wreckage and then kills him.

Jack has been dismissed from CTU and now works for Secretary of Defense James Heller. Jack is involved with Heller's daughter, Audrey, and their relationship is growing serious. Jack is in LA for a meeting with CTU Director Erin Driscoll, who fired him a year ago, but he finds himself alienated at CTU by Driscoll and the new staff when reports of a possible terrorist strike come in.

A young computer hacker named Andrew Paige sees something that he shouldn't on the Internet. Suddenly, he finds himself running for his life as he is pursued by terrorists. Luckily, he is able to warn Chloe O'Brian before its too late.

The Turkish Araz family are not what they seem and it becomes clear that they are heavily involved with the day's events. Meanwhile, Heller has a confrontation with his estranged son.

Jack finds that Chloe is his only ally as he tries to prove that the train explosion is merely a diversionary target.

Jack pushes Driscoll to reinstate him in the wake of Heller's kidnapping. He finds himself working as a subordinate when the Field Ops team goes to pick up Andrew.

Behrooz finds himself torn between his loyalty to his parents and his feelings for his girlfriend as he is sent on his first assignment.

Heller tries to comfort his daughter as his captors broadcast their intentions on the Internet.

Jack tracks Kalil with Chloe's aid, but is forced to watch helplessly as Andrew is tortured for information. Behrooz pleads with his parents when they order him to bring Debbie to their home for questioning. Curtis uses harsh methods to interrogate Heller's son as Driscoll continues to search for Jack.

Jack acts as a bandit and takes hostages to buy Chloe more time to get satellite surveillance. Meanwhile, Chloe must confide in Edgar for help when Driscoll becomes suspicious. Behrooz discovers just how far his mother is willing to go to insure that their secret mission is successful. Heller and Audrey attempt to escape from Omar, while Driscoll uses her influence to deal with a personal crisis.

Jack initiates a fierce gun battle with the terrorists as he races to rescue Heller and Audrey before the military strike begins.

Dina questions her own dedication to their cause when her husband reveals his plans for Behrooz.

Driscoll and Curtis uncover new evidence that points to an even bigger attack by the terrorists.

Audrey has an awkward meeting with her husband, and Heller butts heads once again with his son.

Jack takes Audrey to ID the man she recognized at the compound, but the terrorists get there soon after and pin them down. Jack must call on an old friend to help him escape.

Heller puts Edgar in charge of dealing with the nuclear override, while Driscoll's daughter takes a turn for the worse.

Dina is wounded when she helps Behrooz flee from Navi. Navi's boss orders him to kill his wife and son before the day is over.

Jack finds that his efforts to bargain with Marwan for the location of the missile strike are futile.

Heller pushes his son for information when CTU discovers that he may have had contact with Marwan before the attacks.

Logan finds himself in the middle of a power struggle between Palmer and the ambitious Speaker of the House.

Tony and Michelle explore their rekindled passion for each other, but their hopes for the future may be short-lived when a familiar player enters the game and takes a hostage.