Jack and Chase quarrel over Chase's relationship with Kim. Meanwhile, they learn of a lead to the possible whereabouts of the virus after raiding a crack-house.

Linda breaks up with Kyle when she learns that he is planning to make a drug delivery.

Wayne approaches Palmer with information about his girlfriend that could damage his chances for re-election, while a CTU employee is not what he seems.

Jack's efforts to break Ramon out of prison go awry when they are captured by a group of inmates during the riot.

Chloe unwittingly stumbles on to Jack's drug problem and she is forced to reveal his secret to Kim and Michelle.

Palmer begins his debate with Keeler, while Kyle prepares to sacrifice himself to save Linda.

Jack and Ramon struggle to evade the authorities, while Palmer must shut the debate down because of the prison break.

Kim accidentally stumbles on to Gael's secret communications, but she isn't able to warn Michelle in time.

The Singer family learns a startling piece of news about Kyle's health.

Tony works to free Jane from CTU before Saunders can harm Michelle, but Chloe and Jack become suspicious of his activities.

Sherry meets with Palmer's political rival and proposes a deal to blackmail him.

Jack and Tony clash as the time for the hostage exchange draws near.

Keeler meets with Palmer and demands his resignation, while Wayne begins a secret operation against Sherry.

Jack pushes the envelope in an attempt to force Saunders to disclose the location of the missing vials.

Sherry meets with David to discuss her terms, while Wayne breaks into her home to find the incriminating evidence.

Tony finds himself facing a harsh penalty because of his efforts to save Michelle.

Jack and Chase must make a powerful sacrifice when dealing with the virus.

Tony is able to help Jack one last time before he is taken into custody, while an angry victim seeks vengeance on Saunders.

Palmer makes a surprising career choice in the wake of Sherry's death.

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 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.

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 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.

Michelle calls on Chloe for help when Edgar begins to crack under pressure. Meanwhile, Audrey stands by as a gravely wounded Paul is taken into surgery.

Marwan's plan continues as his henchman uses an unwilling pilot to gain access to an airfield.

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.

Tony becomes suspicious of Michelle's personal life, while Audrey reunites with Paul and learns the extent of his injuries.

Anderson closes in on his quarry, as President Keeler searches for words that will comfort the American public.