Three years have passed since the events of Day Two. A van deposits a dead body outside the LA Health Unit. The body is infected with a deadly virus. Tony, now in charge of CTU, learns that this is a blackmail tactic to get the US government to release a drug lord, Ramon Salazar, whom Jack has just brought down, before Salazar's brother releases the virus into LA.

Jack and his new partner, Chase Edmunds, prepare to sign a deal with Salazar at the prison. Salazar delivers a mysterious warning to Jack before killing his lawyer.

Palmer is alive and well, and has returned to LA for the first time since the attack three years ago. He is running for re-election and is preparing for an important debate with a rival senator. He is accompanied by his brother, who happens to be his chief of staff and his doctor, who also happens to be his girlfriend.

A young boy, Kyle Singer, is seen examining a large, mysterious bag of white powder. He plans to deliver it to some dealers later that day, but he is being watched by Salazar's men.

Kim now works at CTU and has been dating Chase for three months. Kim wants to tell her father, but Jack has a mysterious health problem that may make things more complicated.

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 and Hector both search for Kyle, while Chase finds himself out of action at CTU.

Wayne and Palmer argue over the implications of Anne's past, while Ramon takes out one of his opponents in a surprise move.

Tony leads an operation to get Kyle, but tragedy strikes when Salazar's man gets there first.

Michelle is torn by personal and professional obligations when she must take charge of CTU after Tony is wounded.

Kyle and Linda are captured by Hector's men.

Jack and Palmer have a secret conversation, leading Jack to take drastic action in the face of the Salazars' threat. But Jack's plans are complicated when Chase gets in the way.

Anne pushes Palmer back onto moral ground in the face of her ex-husband's blackmail scheme.

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.

Ramon holds Jack hostage as he flies to Mexico. Ramon plans to kill Jack, but Jack manages to hold him at bay until the plane lands. Ramon then learns that Jack and Hector are working together to break him out of prison.

Gael is captured while trying to escape from CTU. Chappelle interrogates Gael but is interrupted by Tony, who claims that Gael is part of a bigger plan.

Anne is given a chance to clear Palmer's name, while Chase heads for Mexico to rescue Jack.

Palmer and Chappelle view a message from Jack, explaining his undercover work with the Salazars. Meanwhile, Hector tries to convince Ramon that they should deal with Bauer so that they can get their hands on the virus.

Jack has a private moment with Claudia and she blasts him for leaving her behind when he arrested Ramon.

Chase arrives in Mexico, but is captured by Hector's men. Ramon devises a way for Jack to prove his loyalty to them.

Anne manages to clear Palmer's name, but she breaks off her relationship with him.

Palmer clears Anne's name with the press, but he finds that he is facing a new problem when his biggest campaign contributor reveals a startling secret about Wayne.

Kim and Michelle worry about Tony's mental ability when he continues to make mistakes.

Jack and the Salazars meet with Amador, but the deal turns sour when Nina Myers appears on the scene and competes for the virus.

Claudia manages to free Chase from his captors, but she pays a high price for her efforts.

Wayne turns to Julia, while Palmer calls on Sherry for help against Milliken.

Jack and Ramon make plans to get to Nina, in the hopes of intercepting the virus.

Chloe's work at CTU is disrupted when she is forced to take care of her baby.

Jack forces Nina to work with him to get the virus from Amador.

Palmer meets with Sherry and convinces her to help him deal with Milliken.

Chase remains in Mexico to help with the capture of the virus.

Ramon and Hector quarrel when Hector wants to pull out of the deal.

Things finally come to an explosive head as Jack, Ramon, Nina and Chase all close in on the virus. But the deal takes an unexpected turn when Amador pulls a fast switch.

Kim becomes suspicious of Chloe when her story about her baby doesn't check out.

Sherry finds a witness who can damage Milliken, but her advantage is short-lived.

Jack and Chase interrogate Nina, but the worm turns when she causes havoc with the CTU computer systems.

Chappelle puts Chloe under guard when she refuses to answer questions about the baby, but is forced to release her when she is the only one who can deal with the crisis.

Sherry takes her battle directly to Alan, forcing Julia to act as her accomplice.

Tony interrogates Nina about Alvers' location, while Chappelle orders Jack to undergo a medical examination in light of his drug use.

Kim and Chase have a conversation about the baby.

Palmer confronts Sherry about Alan Milliken's death, while Julia begs Wayne for help.

Chappelle threatens to pull Jack from the investigation for killing a material witness, but Jack persuades him to hold off when they find a fresh lead on Amador's whereabouts.

Amador and Alvers meet with the man who is planning to unleash the virus in LA within the hour.

Michelle and Gael locate the target area, but things really hit the fan when they attempt to capture Alvers in the process of releasing the virus.

Wayne argues with Sherry over the consequences of Milliken's death as the police turn up the heat on Julia.

Jack and Chase track Amador in the hopes that he can lead them to the man behind the virus attack.

Michelle faces a mob riot when the hotel is quarantined.

Sherry extorts Palmer, forcing him to manufacture her alibi for the police.

Saunders begins making demands on the President. He forces Palmer to hold a strange press conference and then orders the death of the CTU agent who can trace him.

Jack learns of a woman who can lead him to Saunders, but a surprise attack shifts the balance of power.

As Gael's symptoms get worse, Michelle devises a way to relieve the suffering of the victims at the hotel.

Jack and Chase race against time to capture Saunders before the deadline, while Chappelle plans an escape.

Michelle begins a frantic search when she learns that an infected hotel guest may have slipped through her fingers.

Jack reluctantly works with Kim on an undercover job that may help them gain an advantage over Saunders.

Palmer informs his cabinet of the crisis, while Saunders makes yet another demand.

Chase tracks down the man who left the hotel and learns that the virus is spreading to the general public.

Jack attempts to use Saunders' daughter as leverage to gain control of the virus, while Palmer must deal with the discontented press.

Michelle gets the results of her test, while Adam finds that the virus has struck too close to home.

Tony finds himself compromised when Saunders exploits his only weakness.

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 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.