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.

While Green pressures Nina and Tony to reveal Jack's whereabouts, York leads Jack to Gaines' compound where Jack attempts to rescue Teri and Kim without backup from CTU. Meanwhile, Senator Palmer's plan to prevent a murder fails and he begins to suspect his wife may have a hand in the conspiracy.

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.

During their escape from the safe house Teri and Kim become separated, and the traumatic events of the day result in Teri passing out and then awakening with amnesia. Assuming her mother has been re-kidnapped, Kim conducts her own search for information concerning her mother's abductors. When the plan to track Alexis backfires, Jack pretends to be Alexis in order to keep a rendezvous with one of Alexis' contacts.

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.