18 months after Jack Bauer faked his own death to avoid a worse fate, he's pressed into service when President Palmer is assassinated, Dressler is killed and Tony is wounded by a car bomb and Chloe is on the run.
Jack investigates the scene of Palmer's assassination with the help of Chloe. President Logan plays a recording for Martha that indicates that Palmer's call to Martha wasn't as important as she believes.
From a hidden location, Jack feeds CTU information about the airport terrorists. Martha searches for proof of the actual dialogue she shared with Palmer and finds a hard copy. Walt informs the terrorists that Palmer is hiding in the airport and they threaten Derek's life to draw him out.
The terrorists use captured Jack to try and run the CTU agents ready to storm the airport into a trap. A new person, Lynn McGill, arrives at CTU and takes over from Buchanan, creating waves, but ultimately, through diligence, realizes that Jack is sending them a secret message.
Curtis finds the empty stash and learns that the terrorists have nerve gas in their possession. Audrey interviews Diane at Lynn's direction. Chloe begins to suspect Spencer's activities at CTU and while she interrogates him with Buchanan, an attempt is made on Jack's life by a man Spencer gave access to. Spencer reveals he has been working for the White House. President Logan decides to commit his wife, but she flees.
Jack decides to meet up with Mike to alert him to Walt's involvement in the threat, but the call is also intercepted. Knowing that Jack is on his way, Walt confronts the President with news that Walt was involved with the terrorists.
President Logan demands that Jack stay on the trail of the terrorists. Lynn's sister Jenny calls asking for his help. Ivan contacts a man about gaining control of the container detonation and CTU traces the call to a downtown Los Angeles building. Jack takes down Ivan's contact who demands a deal in order to help. Lynn is ambushed and robbed when he steps out to meet his sister.
Jack tries to pass himself off to the Russians as the person who can fix the gas canisters. Novick approaches President Logan with the idea of using the suicide of Walt Cummings to cover up his ties to the terrorists.
McGill orders Jack be brought into custody for disobeying orders and not helping the terrorists at the mall. Jack enlists Audrey's help when Nathanson calls demanding to speak with Jack.
CTU comes across information that leads them to believe that terrorists might attack the Russian President's envoy. Lynn McGill refuses to pass this information along and Audrey tries to circumvent his authority.
Jack is reconnected with Christopher Henderson, a rogue agent who brought him into the counterterrorism unit over ten years earlier. Jack must interrogate Henderson's wife, Miriam, about her dealings with terrorists. Tony awakens and discovers what has happened with Michelle, while Lynn tries to regain his access card. Meanwhile, Curtis discovers that a gas attack has occurred at a nearby hospital.
Henderson and Kim have an icy face-to-face when Jack takes him to be questioned at CTU. Logan is told to enact martial law in Los Angeles by the Vice-President, which prompts Mike to try and reason with Martha for assistance. Meanwhile, Lynn's disappeared access card may have allowed a terrorist breach to occur.
CTU is forced to take some alternate measures when the new government and CTU members create additional difficulties in thwarting the terrorist plot.
As the terrorist threat draws ever closer, Jack is forced to verify the truth behind some staggering new information.
Jack tries to determine Audrey’s possible allegiance with the terrorists. Meanwhile, Bierko prepares to spread his nerve gas across the city, while Aaron and Wayne learn that Henderson has put a hit on them.
Jack and Wayne manage to learn information about the murder of Palmer that is tied to Logan’s camp, but must try and get past Henderson’s protective wall of silence which surrounds Logan. Meanwhile, Audrey must pick amongst Buchanan and Chloe when Homeland Security takes over.
New findings regarding the day's terrifying events keep agent Jack Bauer away from CTU, whose plans Homeland Security is ordered to reprioritize. Chloe and Audrey team together to assist with field operations as the investigation takes a sudden turn.
Martial law is still enforced, Jack, Audrey and James Heller get reacquainted, President Logan feels the country is no longer in danger of attack and CTU remains under pressure in regards to finding remaining fugitives.
President Logan continues to face difficulties, while Jack Bauer has new evidence. Jack's interrogation proves to be effective; however his methods for conducting that interrogation may have consequences.
Homeland Security leaders Karen Hayes and Miles Papazian respond to new developments while former CTU commander Bill Buchanan works with analyst Chloe O'Brian to assist Jack Bauer. As this long day turns into the middle of the night, the feelings at the presidential retreat escalate to new heights for the First Lady.
As Agent Jack Bauer inches closer to bringing the culprits to justice, President Logan takes unthinkable and shocking measures to impede Jack's progress. With CTU commanders Bill Buchanan and Karen Hayes covertly working together, a counterproductive dynamic is created.
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.
As the clock approaches the final hours of an extremely terrifying day underscored by a Presidential assassination and the threat of nerve gas, Agent Jack Bauer seeks to avenge the painful loss of innocent lives as well as CTU beloveds Edgar Stiles, Tony Almeida, Michelle Dessler and newcomer Lynn McGill. Meanwhile, shocking developments are uncovered as the investigation takes an unbelievable turn.
In part 2 of the seaosn finale, as the clock approaches the final hours of an extremely terrifying day underscored by a Presidential assassination and the threat of nerve gas, Agent Jack Bauer seeks to avenge the painful loss of innocent lives as well as CTU beloveds Edgar Stiles, Tony Almeida, Michelle Dessler and newcomer Lynn McGill. Meanwhile, shocking developments are uncovered as the investigation takes an unbelievable turn.