It is California Presidential Primary Day in Los Angeles and the longest day of Federal Agent Jack Bauer's life is about to begin. Jack has just patched his family back together after a trying separation. Midnight finds Jack playing chess with his teenage daughter Kim. As she goes to bed, Jack gets a call from Nina Myers, his coworker at the Los Angeles Counter Terrorist Unit, telling him to come in for an emergency meeting. Jack has no choice but to report in, but he and his wife Teri learn that Kim has snuck out before he leaves. Kim and her friend Janet York drive to an empty furniture store where they party with a couple of college guys whom they recently met. Kim enjoys herself at first, but starts to become suspicious of the two men when they won't take her home. At CTU, Jack learns that an assassination attempt is being planned against Senator David Palmer; the first African-American presidential candidate with a serious shot at the White House. Jack's boss and friend, Richard Walsh, warns him that people in their own agency may be working with those who plan to kill Palmer. Senator David Palmer is making his plans for Primary Day when he receives a strange phone call that puts him in a bad mood. Jack shows how far he's willing to go to do his job when he blackmails District Director George Mason in the hopes of learning information about the Palmer hit. Teri receives a call from Janet's father, Alan York, and the two decide to go out looking for the girls. A European photographer makes pleasant conversation with a beautiful woman on a plane bound for LA. The two end up in a lustful encounter, but more sinister plans come to light that involve a stolen wallet and the destruction of the airplane.

Jack's plans to find Kim are sidetracked when he gets a call from Walsh, who is being stalked by assassins connected with the Palmer hit. Jack comes to Walsh's rescue and Walsh gives him a key card containing valuable information. Palmer learns of the possible threat against his life, but slips away from his security in order to learn more about a damaging story that is about to hit the press. Dan and Rick start to become violent when Kim refuses to make a phone call for them. Mandy meets with the man behind the Palmer hit, but her partner holds things up when she demands more money for the ID.

Jack learns the identity of the agent who may be working against him. He acquires Jamie's help to get evidence. Palmer finally learns of the damning story about to hit the press, involving his son and a possible murder several years ago. Kim and Janet manage to escape their captors. Kim is able to get a message to Teri, but an accident results in her recapture. Tony calls for outside help when he decides that Jack should be relieved of his command.

Jack breaks out of a locked down CTU so he can follow a lead from the key card. George Mason puts pressure on Jamie and Nina to learn of Jack's whereabouts. Palmer discovers who the source is that leaked the story about his son. Rick and Dan argue about how to handle Janet, while Teri and Alan run into an obstacle as they try to help their daughters.

Jack takes drastic action to gain the cooperation of the only man who knows how to save Kim, but Mason's interference threatens to ruin Jack's advantage. Kim is delivered to Gaines and she learns just how ruthless he can be when he's crossed. Meanwhile, Teri and Alan watch over Janet in surgery. Palmer talks to the reporter who plans to break the story about Keith and she shows him proof to back up her claims.

Jack and Teri are reunited at the hospital. Jack plans to question Janet when she wakes up, but he is interrupted when Gaines contacts him and begins forcing Jack to follow his orders. Palmer is angered when he learns that his family went to great lengths to conceal the truth from him. Kim tries to persuade Rick to help her escape from Gaines. Teri receives a message from Nina and discovers that she is in more danger than she realized.

Jack is now under Gaines' control and is forced to sabotage operations at CTU. Nina becomes suspicious of Jack and gets in his way. Teri pulls a surprise attack on her kidnapper, but she makes the wrong move and finds herself in more trouble than ever. Palmer tries to persuade his family to go public with the story before the press can report it.

Nina and Tony discover the identity of the mole who is working for Gaines. Teri and Kim are finally reunited. Still under Gaines' control, Jack goes to the Palmer breakfast, where he learns what Gaines has planned for him. Jack manages to foil the assassination attempt, but is captured by Secret Service in the process.

Jack tries to explain his situation to Secret Service, but they are unwilling to release him. He manages to escape and hides out with a hostage while contacting CTU. Nina and Tony turn up the pressure on Jamey and finally learn about Gaines, but tragedy strikes before they can act. Teri sacrifices herself to save Kim from a brutal experience.

Teri manages to call CTU using a stolen cell phone. She and Jack have an emotional conversation, but her signal dies before the call can be traced. Nina and Tony find themselves facing a new boss, Alberta Green, as they continue to secretly help Jack. Palmer's political associates begin to sweat as Carl plans to take serious action against Keith's therapist. Jack uncovers a new name from the key card that may lead him to Gaines.

Palmer begins to form a connection between himself and Jack Bauer. Gaines meets with Drazen, the man behind the Palmer assassination plot. Drazen gives Gaines an unpleasant ultimatum that will result in the deaths of Teri and Kim. Jack's interrogation of Cofell goes awry, but he discovers another familiar face who can lead him to his family.

Jack sneaks into the compound and rescues his family, but Gaines catches on to him and gives chase. Jack and his family are attacked by Gaines and his men, causing them to become separated. Palmer tries to talk to Keith's therapist, but discovers that Carl has been there before him. Green tries to turn Nina and Tony against one another.

Jack plays a deadly game of cat and mouse with Gaines that results in a final confrontation. CTU arrives on the scene and flies Jack and his family to safety. Palmer plans to talk to the D.A. about Ferragamo's murder, but a threat from Carl changes his mind. Nina discovers that a second assassin has arrived in LA, which means that Palmer's life is still in danger.

Jack is forced to leave his family when Green places him under arrest and holds him at CTU. Once there, Jack must explain his actions to District Director Ryan Chappelle. Nina accompanies Teri and Kim to a clinic, where they are given a medical examination. Nina becomes suspicious of a phony FBI agent and decides to move the women to a safehouse. Palmer goes to CTU and demands a meeting with Jack.

Jack and Palmer finally meet face to face and learn of their connection through Operation Nightfall. They work together to track down those who are seeking revenge on them. A member of Palmer's staff has a romantic encounter with the would-be assassin. Teri and Nina have a revealing conversation about Jack at the safehouse.

Keith takes matters into his own hands when he arranges a secret meeting with Carl. Elizabeth realizes who Alexis really is and immediately comes forward. Jack and Mason question her and then convince her to aid in his capture. The safe house is attacked, but Teri and Kim manage to escape. They are then separated by an unforeseen accident.

Teri is struck with amnesia and must turn to an old friend for help. Kim goes to Rick's house in hopes of finding a lead. Jack sets Elizabeth up for a sting against Alexis, but she pulls a fast one at the last second.

Palmer and Sherry clash over the handling of the evidence against Carl. Kim finds herself in a sticky situation with Dan's brother. Jack poses as Alexis and meets with a stranger who has vital information, but an agent with a grudge from Jack's past picks a bad time to get even. Teri and Dr. Phil return to the Bauer home, putting themselves in further danger.

Jack and Mason investigate the area where the power was to be turned off. Nina tries to warn Jack about his family's disappearance, but Mason hampers her efforts. Teri explores her home in hopes of regaining her memory as Drazen's man closes in for the kill. Kim finds herself in the middle of a botched drug deal, while Palmer breaks the story about Keith to the press.

Kim lands in jail when the cops don't believe her story. Jack stumbles onto a secret detention center. He arrives just in time to bluff Andre and his men out of freeing a prisoner who is being transferred in. Jack confronts the prisoner and discovers that he knows him from the past. Before Jack can learn any more, Andre and his soldiers make another run on the prison.

Palmer is relieved when it seems that he has dominated in the California polls in spite of his crisis. Jack becomes locked in a stalemate with the Drazens. They manage to turn the tables and take Jack as a hostage as they escape. Kim is finally released from jail, but her freedom is all too brief when she is again abducted by terrorists.

The Drazens contact Mason and offer to trade Jack for Alexis. Mason is reluctant, but Palmer pressures him to go ahead with the deal. Nina and Tony try to distract Teri when they learn of Kim's abduction. Palmer is tempted by a member of his staff.

The Drazens threaten to kill Kim unless Jack pays Palmer a visit. Jack complies and is able to save Palmer's life once again. Jack then agrees to trade his life for Kim's since the Drazens believe Palmer is dead. Kim manages to escape from the Drazens and swims to safety. Teri tells Jack about her unexpected good news. A second mole is revealed inside CTU.

Palmer has had enough of Sherry's power plays and has a final showdown with her in the hotel ballroom. Nina knows that the Drazens have no leverage without Kim, so she calls Jack and tells him that Kim's body was found floating in the harbor. Jack is crushed by grief and then he flies into a rage. He rushes the Drazens and a bloody shoot-out ensues. Nina's cover is blown and she makes a final dash for safety, leaving a bloody trail behind her. Jack is able to apprehend her before she can escape, but he is stunned to learn that he didn't arrive soon enough.