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 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.

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

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

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.

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

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.

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

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.

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.

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

Jack learns the identity of a man who may be able to lead him to Marwan. He forces Dina to help him in a mock hostage situation that will get him close to the suspect.

Marwan decides to trade Jack for Behrooz in hopes of distracting CTU from his real objective.

CTU raids Marwan's hideout, but he manages to slip through their fingers. Luckily, Jack locates a lead that may help him identify Marwan's next target.

Mike pushes for the Vice-President to be sworn into office as the White House waits for news of Keeler's fate.

Curtis brings in a suspect who has critical information about Marwan's next strike. Michelle orders Curtis to use harsh methods to interrogate the suspect, but things get complicated when Marwan sends a lawyer to defend his rights.

Chloe is unexpectedly thrust into fieldwork when a woman reports that her boyfriend may be part of the terrorist conspiracy.

After Logan authorizes Palmer to take charge of the crisis, Palmer sanctions Jack and Curtis to infiltrate the Chinese Embassy on a covert operation to capture a suspect.

CTU scrambles to forge an alibi when a Chinese representative produces evidence of American involvement in the raid on their embassy.

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

Mandy uses Tony as leverage to try to force Michelle to aid in her escape.

As the missile speeds toward its destination, Mandy cuts a deal with Jack and the President to give up Marwan's location in exchange for a full pardon on all of her crimes.