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 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 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.

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

Tony and Michelle weigh the consequences of the trade, while Curtis reluctantly interrogates Behrooz in order to learn more about Marwan.

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.

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

Two innocent bystanders are dragged into the fight as Jack and Marwan both pursue a briefcase that contains the launch codes for America's nuclear arsenal.

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.

Logan's advisors begin to worry as he continues to exhibit fear in the face of the ongoing terrorist threat.

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

Jack and Curtis find their latest efforts to trap Marwan thwarted by the Secret Service, who have orders to arrest Jack for insubordination.

Mike calls on ex-President David Palmer for help as Logan continues to display a lack of leadership.

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.

Tony and Michelle continue to bump into each other's personal lives, while Buchanan expresses anger at being kept out of the mission.

Jack faces Audrey's wrath when a medical dilemma places Paul's life in jeopardy.

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

Audrey gives Jack the cold shoulder, while Palmer and Logan clash over the consequences of Palmer's decisions.

Jack and Curtis close in on Marwan as the countdown for the missile launch moves ahead.

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.

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

A Chinese security operative captures and interrogates one of the CTU agents involved in the raid on the embassy.

Palmer and Logan argue over the option of evacuating all major cities that may be targets for the missile.

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.

Tony and Michelle share a happy reunion, but Jack finds his relationship with Audrey shattered as she makes her feelings known.

Logan's advisor suggests a drastic course of action when the Chinese government demands that Jack be handed over to them for trial. Palmer tries desperately to warn Jack of the danger against him.

Jack's life changes forever as Tony, Michelle and Chloe help him to evade the trap set for him by Secret Service.

The Russian and US Presidents are set to sign an anti-terror treaty when assassinations rock Los Angeles, a call from a distressed Chloe forces Jack into action and President Logan's wife believes there may be a conspiracy.

Jack and Chloe infiltrate Wayne Palmer's penthouse where David Palmer was assassinated, in the hopes of finding a clue to his murder. Jack convinces Wayne to cooperate and finds some information in Palmer's memoirs that leads him to the airport where the Russian president is about to land. CTU and Secret Service attempt to capture Jack, but they only find Chloe as Jack and Derek slip through their grasp. Martha tries to convince her husband that she received an important phone call from Palmer the night before his death, but a tape of their conversation seems to contradict her claims. Jack goes to the airport to find a lead, but a group of renegade Russian terrorists take hostages, including Derek, before he can act.

Jack conceals himself inside the airport hanger and surveys the Russian separatists, feeding information to Curtis and his assault team.

The terrorists begin executing hostages in an attempt to make Logan cave into their demands.

Martha discovers that all traces of her conversation with Palmer may not have been erased.

Jack is captured by the terrorists, who use Derek as leverage to force him to compromise CTU's assault operation. Logan refuses to give into the terrorist demands as the time of the co-signing of the treaty draws near. Martha tries to tell her husband of the evidence she uncovered about her phone call with Palmer, but Walt interferes with her efforts. A new representative from District arrives at CTU and begins to undermine Buchanan's authority.

Jack returns to CTU and has an emotional reunion with Audrey, while Curtis tracks the Russians and the stolen nerve gas.

Walt sends in a hit man to silence Jack.

Chloe discovers that Spencer has his own agenda at CTU.

Walt convinces Logan to have Martha institutionalized when she tells them of the attack on her.

Jack contacts Mike and asks for a secret meeting in hopes of gathering evidence against Walt.

Martha begs Aaron for help, while a desperate Walt tries to force Logan to aid him in his complicity with the terrorists.

Buchanan butts heads with Lynn over a decision to violate the president's orders, while the Russians attempt to escape with the nerve gas.

Jack tracks down a man who has a connection to the terrorists. Logan is forced to grant him immunity in exchange for information on their whereabouts.

Mike and Martha argue over Logan's public response to Walt's involvement with the Russians.

Lynn is drawn away from his post by a personal crisis as the terrorists come closer to striking against the U.S.

Jack poses as Rossler and manages to trace one of the canisters of nerve gas, but he goes against orders from the president when the terrorists target a shopping mall.

Logan and Martha quarrel when she learns of her husband's intention to cover up the real reason for Walt's suicide.

Lynn orders Curtis to take Jack into custody for violating presidential orders, but Jack breaks away from Curtis when Nathanson contacts him through Audrey and offers information on the location of the nerve gas.

Lynn begins to unleash on his coworkers as he discovers that they may be working against him to aid Jack.

The head of the terrorists makes a demand on Logan, and Martha takes a dangerous step to get her husband's attention.

Logan wrestles with his conscience when he learns that Martha is riding in the motorcade with the Suvarovs.

Audrey convinces Curtis to take action as Lynn continues to abuse his authority.

Jack learns that an ex CTU agent, Christopher Henderson, works for a company which is connected with the nerve gas.

Jack interrogates Henderson's wife about her husband's connection to the terrorists.

Lynn begs his sister to return his stolen keycard, while Tony regains consciousness and learns of Michelle's fate.

Martha gives Logan the cold shoulder, while Curtis tracks another nerve gas attack to a local hospital.

Jack takes Henderson to CTU for questioning. Once there, he faces an unhappy reunion with his daughter, Kim.

Mike appeals to Martha for help as Logan faces pressure from his Vice-President to declare martial law on L.A.

Lynn connects the theft of his keycard to an impending strike on CTU, but his warning comes too late to stop a terrorist from infiltrating the building.

Jack and Lynn must take action when they discover that the nerve gas is breaking through the protective seals that are keeping the CTU survivors alive.

Chloe wrestles with her grief over Edgar's death, while Tony contemplates revenge against Henderson.

Martha bolster's Logan's spirit as he wavers under more political pressure from Gardner.

Jack bargains with a German intelligence agent in order to find the woman who can lead him to Bierko.

Buchanan chafes against the new chain of command as Homeland Security prepares to take over CTU operations.

Aaron receives a request for a secret meeting from Wayne Palmer, as Logan implements martial law in L.A.

Jack pushes Audrey for answers when evidence indicates that she may be involved with the terrorists.

Wayne and Aaron flee from a group of gunmen who have orders from Henderson to kill them.

Bierko and his men gain access to a natural gas plant, where they plan to distribute the nerve gas throughout L.A.

Jack and Wayne locate a source inside Logan's administration who has information on Palmer's assassination. But things get complicated when they learn that Henderson is using leverage to keep her silent.

Audrey is forced to choose between Chloe and Buchanan as Homeland Security begins to absorb CTU.

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.

After stowing away aboard the flight containing Henderson's contact and the incriminating recording, Jack finds himself having to deal with a pilot who refuses to cooperate.

Martha begins to break down, and Mike becomes suspicious of what she is hiding from him, while Logan tries to calm his financial backers.

Jack searches for a safe place to land the plane when Logan gives orders to shoot it down. Hayes readies Bierko for a transfer to a secure facility, while Audrey receives some good news about her father. Logan becomes desperate as his plans start to unravel, but an unexpected ally at CTU turns the tables in his favor.

Bierko escapes from the transport taking him to Division and uses his last canister of nerve gas for a surprise strike. Jack is forced to work with Henderson to find Bierko, but Henderson wants something in return. Logan takes drastic steps to deal with Aaron when he won't go along with the cover-up.

Jack and Henderson wage an assault on the submarine as Bierko prepares to launch an attack against L.A. Chloe brings in someone from her past to aid her. Martha and Aaron enlist Mike's help in their plans to expose Logan.

Jack takes his fight directly to the President as he attempts to force a confession from him that will prove his involvement in the day's events. Martha vents her hatred for her husband in front of the press as Logan gives a eulogy over Palmer's body. Jack and Audrey share a tender moment when it seems the crisis has subsided, but Jack finds that his problems have only begun as his past catches up with him.

A new U.S. President has negotiated Jack's release from a Chinese prison so Jack can stop a series of terror attacks... with his life.

Can Jack and an unlikely ally stop a suicide bomber and track the architect of these acts of terror?

Jack and former terrorist leader turned peacemaker, Hamri Al-Assad, track a suspicious man to a self-storage facility.

While Jack has problems with Curtis, CTU discovers a nuclear threat and raids a terrorist warehouse, and a shocking explosion shakes LA.

Four more "suitcase nukes" are out there, and Jack's only lead takes him to the home of his brother.

Jack finds himself confronting his father while the lead terrorist needs help if he's to explode the remaining bombs.

Family troubles: Jack with his brother and father while Morris worries about his own brother.

The President meets with a known terrorist, while Jack searches for Morris, Fayed seeks a trigger, and Tom Lennox makes a choice.

Jack looks for a Russian national as his father plots with shadowy patriots. Meanwhile, the President and the Constitution may be in danger.

While Jack looks for Gredenko and tries to save Josh, White House intrigue deepens.

Jack's search takes him to the home of an old enemy, while a bomber slips into the White House.

Jack's search takes him to the Russian Consulate, while the Vice President contemplates a sweeping executive order.

The search for Gredenko finds Jack hunted inside the Russian Consulate and a former President calling on his ex-wife.

CTU's satellite tracking system is compromised as a bomb-carrying drone heads toward an unknown city. Meanwhile, US forces prepare for a nuclear retaliation on the Middle East.

Jack enlists the help of an autistic man in the search for Gredenko, and Sandra Palmer makes a fateful decision.

Jack baits Fayed with Gredenko, while the President faces a challenge to his office.

To find the remaining suitcase bombs, both Jack and the President set up bluffs.

The Chinese want Jack to give them Russian nuclear technology, and the President wants the Vice President's resignation.

Jack goes rogue to save a woman, and the White House needs a fall guy for Fayed's success.

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.

Russian ire worries the White House, while the Chinese take on CTU, and the damaged circuit board is in surprising hands.

Russia presses the White House, while Jack must try to thwart the Chinese and his oldest antagonist simultaneously.

In order to prevent a war with Russia, the White House orders a trade of Jack's nephew for the circuit board, and puts Jack in custody so he cannot stop the exchange.

Jack rushes toward his nephew and his oldest antagonist, as a Chinese sub and F-18s race for the same destination. Chloe faints. Can Jack find any peace?