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 and Driscoll agree to work together when the police make a critical error in dealing with Kalil.

Heller and Audrey take drastic action in the hopes of avoiding the impending trial.

Driscoll makes a deal with Chloe to get her out of the way, while the Araz family faces another problem in their cover-up.

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 and Audrey hide out with an embittered Tony as Audrey makes a positive ID of the man from the kidnapping.

Marianne successfully diverts suspicion from herself, leading Driscoll to torture the wrong person for information.

Behrooz takes Dina to a hospital for medical attention, but the doctor becomes suspicious of her gunshot wound and phones the police.

Jack and Tony are able to trace Dina's location, but Behrooz slips through their fingers. Dina demands a full pardon for her son in exchange for information on the override device.

Edgar wrestles with his emotions when his mother is trapped in the path of the nuclear fall-out.

Behrooz turns to his uncle for help, but Navi closes in on him before he can get away.

Audrey places herself in a compromising position when she and Jack learn of her husband's possible involvement with the terrorists.

Marianne offers Curtis information on the location of the override device in exchange for a lighter sentence, but the tables soon turn and Curtis discovers that there's another player in the game.

Sarah bargains for a stronger position at CTU, while Tony takes charge of the interrogation of Dina and Behrooz.

Jack and Curtis finally learn the location of the override device. They try to trap the architect of the terrorist strike before the remaining nuclear plants go critical.

Tony pushes Dina to reveal more of her knowledge of Marwan, while Driscoll faces another personal crisis with her daughter.

Jack and Paul visit the defense contractor who employed Marwan, but the company executives put their own interests ahead of CTU's and take major steps to destroy evidence.

After a grief-stricken Driscoll relinquishes her command, Heller appoints Tony as interim director to oversee the latest crisis.

Jack and Paul enlist civilian help when they must fight off soldiers sent to kill them by McLennan-Forster.

Tensions flair between Tony and Michelle as she assumes command of CTU. Sarah tests Michelle's authority by giving her an ultimatum.

Audrey reevaluates her feelings for Jack, while Marwan sets the next phase of his plan in motion.

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.

Jack and Wayne locate a recording that definitively proves Logan's involvement in the terrorist conspiracy, but Henderson corners them in the hopes of destroying the incriminating tape.

Logan tries to cover his tracks by issuing a warrant for Jack's arrest.

Audrey contacts her father for help, while Chloe gives her cover while helping Jack evade CTU.

Jack shows Heller the incriminating recording in the hopes that he will help them take down Logan, but Heller has plans of his own that will save face for the presidency. Miles tricks Chloe into leading CTU to Jack and Audrey. Mike and Martha both become more suspicious of Logan's actions, while Heller confronts the president with an ultimatum.

Jack captures Henderson and attempts to force him to hand over the recording, but Henderson targets Heller to buy himself more time. Logan confesses his involvement in Palmer's assassination to a distraught Martha. Miles traces Chloe to Buchanan's home, but Hayes finds herself questioning Logan's motives as CTU closes in.

A missing computer circuit board: the Russians get wind of Chinese espionage, the White House must find a leak, and CTU interrogates an unlikely witness.