Moments after discovering his daughter, Kimberly, has snuck out of the house, Jack Bauer is called into work. As head of the CIA's Los Angeles Counter Terrorist Unit office he's told that an assassination attempt will be made within the next 24 hours on the life of presidential candidate David Palmer and it's up to his team of agents to keep Palmer alive. Meanwhile, Jack's wife, Teri, sets out to find their daughter.

Teri receives a phone call from Alan York who says his daughter, Janet, is a friend of Kimberly's and is also missing, so the two parents join forces to try and find the girls. Even as Jack is secretly called out of the office and given an encrypted key card containing information on the Palmer hit that could lead him to a dirty agent within his organization, the explosion of a 747 over the Mojave Desert becomes yet another piece in the puzzle.

Kimberly and Janet realize they are in danger after it becomes clear that their two dates have kidnapped them. Despite the threat on his life, Senator Palmer sneaks out of his hotel to meet a reporter who has damaging information that could affect his campaign. Back at the office, Jack has Jamey, a member of his team, decrypt the key card.

Convinced Jack needs to be relieved of his command, one of his team members, Tony, calls George Mason who arrives and puts the CTU under lock-down. With the help of his second-in-command, Nina, Jack slips out and heads for an address listed on the key card. Once there he encounters a shooter who hints that Kimberly's disappearance is part of a bigger plan. Meanwhile, the two boys deliver Kimberly to a man named Ira Gaines.

Jack follows the arrested shooter to the police station in hopes of interrogating him only to find that Mason has arrived and taken control of the situation. But when the shooter says he will talk only to Jack, Jack learns he must break the shooter out of jail in order to learn who has kidnapped Kimberly. Back at the hotel, Senator Palmer learns more about the threatened news leak that could hurt his campaign.

Jack meets Teri and Alan York at the hospital where Janet has been admitted. But their reunion is cut short when Jack receives a phone call from Gaines telling him to follow his instructions if he ever wants to see his daughter again. Back at CTU, a dead body found in a car trunk is identified as that of Alan York. When Nina calls Teri and asks her to pass this information on to Jack, Teri suddenly realizes she is also in danger.

Now, under constant surveillance from Gaines, Jack goes back to CTU where Nina inadvertently gets in the way of Gains' plan. Teri tries to escape from York, but her call to CTU only brings more help for York. Meanwhile, Palmer tries to convince his family that they need to tell the press about a secret personal tragedy from their past before it's too late.

In order to save his wife and daughter, Jack is ordered to deliver a briefcase to a man at Senator Palmer's rally. Realizing this man must be the assassin, Jack struggles to find a way to protect both Palmer and his family. Back at the CTU, Nina and Tony think they have found the traitor within the midst.

Although he prevents Palmer's assassination, Jack is taken into custody where his story of his kidnapped wife and daughter is not believed. Knowing his family will only be kept alive if he is free to help Gaines, Jack knows he must escape. Back at Gaines' compound, Teri and Kimberly struggle to stay alive, while at CTU the traitor reveals that the whole unit has been wired so Gaines can watch their every move.

When a new boss, Alberta Green, takes over the CTU, Nina secretly continues to help Jack locate Teri and Kim while Jack tracks down investment banker Ted Cofell, a man who is mentioned on the decrypted key card. A shocked Senator Palmer learns that one of his associates may be willing to commit murder in order to protect his campaign.

A man named Andre Drazen contacts Gaines. Clearly disappointed that the morning's events did not go as planned he threatens to relieve Gaines of his assignment. Across town, Jack kidnaps Cofell and, after hearing Cofell's Serbian accent, has Nina break into an old case file profiling a secret mission Jack was on in Belgrade and Kosovo two years ago. Then Jack meets Cofell's contact: the man who claimed to be Alan York.

Jack confronts Gaines who, realizing he has failed Drazen's assignment, chooses death over helping Jack. Back at CTU, Nina and Tony interrogate the mother of the traitor and learn of a secret bank account containing deposits from Belgrade. They also learn that additional assassins may be making their way to Los Angeles.

While Jack is questioned at the CTU, Teri and Kim are taken to the clinic to be examined. Meanwhile, certain he recognizes the name Jack Bauer, Palmer has Jack's background investigated. When he discovers that Jack was the sole survivor of a secret mission that he authorized two years ago, Palmer becomes convinced Jack is out for revenge and insists on talking to Jack face to face.

After Jack convinces Palmer he is not trying to kill him, they go over the "Operation Nightfall" file. The goal of the operation was killing a man named Victor Drazen, and a startling piece of information in the file convinces Jack and Palmer that they are both being targeted. Meanwhile, a scare at the clinic results in Teri and Kim being whisked away to a safe house.

Jack learns that one of the assassins, Alexis Mecir, is actually Victor Drazen's son and has become intimate with one of Palmer's aides. In an attempt to find the man ordering the assassinations, the aide agrees to plant a tracking device in Alexis' wallet. Back at the safe house, Teri and Kim discover their location is no longer secret or safe.

Against his family's wishes, Senator Palmer decides to go public about the secret scandal surrounding his family. Even as Alexis' brother, Andre, sends another assassin to the Bauer residence in hopes of finding Teri and Kim, a confused and scared Teri meets Dr. Parslow, a man who says they are good friends. Unaware of the danger Teri is in, he offers to give her a ride home.

Alexis' contact turns out to be a power company worker who was being paid to shut off the electricity to a power grid out in the middle of an empty field. As Jack and Mason follow up on that lead, Kim's search for information lands her in jail, even as Palmer's press conference disclosure could ultimately put his own son in jail.

As Kim tries to get the police to call CTU, Jack discovers that the empty field where the power grid is located actually houses a secret underground government prison and that night's scheduled arrival is none other than Victor Drazen, the man Jack supposedly killed two years ago. Now Jack's convinced Victor's sons are coming to break him out.

Jack's fears are proven true when Andre Drazen breaks into the underground facility to rescue his father. With Jack as his prisoner Victor calls CTU, but Mason tells him he doesn't negotiate with terrorists. Desperate to buy some time, Jack tells Victor that his son, Alexis, is still alive. Back at the police station the police finally let Kim go, but her drive to CTU doesn't go as planned.

Jack has Nina connect Victor Drazen to a hospital phone so he can talk directly to Alexis. Nina then advises Palmer of the situation. Palmer urges Mason to arrange the exchange Drazen proposed: the return of Jack for the return of Alexis. Although Alexis is delivered, Drazen holds onto Kim and tells Jack he needs to do one last thing if he wants to see his daughter again.

Jack arrives at Palmer's hotel with a phone on which Drazen will call and ask Palmer to unfreeze $200,000,000 of his assets. When Palmer tells Jack that Drazen knows that's impossible, Jack grabs the phone and throws it off the balcony. It explodes seconds later. Realizing Drazen will never give up, Jack suggests having the television report that Palmer died in the explosion, leaving Jack to offer himself up in exchange for Kim.

When Jack arrives at Drazen's location, he calls and asks to speak to Kim. Andre angrily refuses and says that they known Palmer is not really dead. Stunned, Jack hangs up and immediately calls Nina to tell her there's still a traitor inside CTU. Momentarily uncertain how to proceed, Jack is suddenly shocked into action when Nina calls back with tragic news.

Jack begins his interrogation of Nina and pushes all boundaries in order to force her to divulge the name of her contact. Palmer is surprised and suspicious when his ex wife Sherry returns and claims to have information about a conspiracy against him from within his own administration. Kim and Miguel plan to kidnap Megan from the hospital in order to take her out of LA and away from the nuclear threat.

Jack forces Nina to help with a sting operation against her contact in the hopes that he can lead them to the bomb. Palmer and Mike try to gage Stanton's loyalty when they investigate the chopper crash. George has a final conversation with his son.

Kim and Miguel are arrested and the police plan to take them back to LA. Tony questions Bob Warner at CTU and learns more about his government connections. Palmer discovers a traitor in his ranks when the journalist whom he was holding is set free. Nina pulls a deadly countermove, setting herself up as the main player in the search to locate the bomb.

George pits Bob against Reza in hopes of learning which one is working with Second Wave. Sherry and Lynne clash over Palmer's speech to the press. Kate is interrogated by Syed Ali, who believes she knows too much about the bomb. Jack and Nina fight off a group of renegade soldiers while waiting to be rescued from the crash site. They nearly escape when Nina takes Jack hostage and demands to speak to the President.

Two traitors are uncovered. One is the mole in the Palmer administration who is working with Stanton. The other is the member of the Warner family who helped Syed Ali to bring the bomb into the country. Jack learns of Ali's location and tries to rescue Kate. Kim and Miguel hatch an escape plan to stay out of LA, but the results are disastrous.

Kate helps Jack to locate and identify Syed Ali. Palmer places Stanton under arrest, but he won't break. Palmer realizes he must go to extremes to make Stanton talk. Kim hits her low point of the day when she encounters a cougar in the woods.

Jack engages in a brutal contest of wills with Syed Ali. George questions Bob Warner to learn more about Marie's motivations for helping the terrorists. Kim is rescued by a mysterious hunter who offers her asylum. Lynne receives a dire warning from Sherry.

Jack and his men prepare for a confrontation with Stanton's renegade soldiers as they converge on the bomb. Kim becomes suspicious of her host, but is forced to stay because of her status with the police. Stanton finally breaks down and gives Palmer some unpleasant news.

Jack and Kate try to break Marie's resolve so they can learn the location of the bomb. Palmer confronts his ex wife with the information that Stanton gave them. George finally steps down as director of CTU and hands the position over to Tony. Kim discovers her host's deception.

Palmer's cabinet invokes the 25th Amendment in an attempt to remove him from power. They place him on trial for his extreme actions during the nuclear bomb threat. Jack and Michelle work against the clock to prove that the evidence against the Middle East is false.

Jack locates the man who forged the audio evidence, but his efforts to break him down are sabotaged by Sherry Palmer, who has a hand in the conspiracy. Kim returns to the Matheson home, where she discovers that her problems are far from over. Palmer searches for an ally who will help him get a message out to CTU. Tony and Michelle take Chappelle out of action when he hinders their efforts to help Jack.

When Jack hits a dead end with his lead, he realizes he must persuade Sherry to help him get evidence against Kingsley. Things go awry at CTU when Division becomes suspicious of Chappelle's absence. Michelle goes off-site to work with Jack. Mike and Prescott ask Palmer to aid them as the bombers get closer to their targets.

Jack and Sherry Palmer prepare for a dangerous meeting with the head of the war conspiracy at the L.A. Coliseum; David Palmer sits in as the Cabinet waits for confirmation of the Cypress Chip's invalidity; the air strike on the Middle East draws nearer; the conspiracy goes to an old agent to put Palmer out of play.

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.

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.