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.

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.

Tony straci łączność z Jackiem. Kim i Michelle martwią się o zdrowie szefa, który przeszedł operację. W Meksyku Jack mówi Claudii o swej misji. Bauer i Salazarowie spotykają się z Amadorem.

Jack i Salazarowie chcą przeszkodzić Ninie w zakupie wirusa. Do transakcji dochodzi w opuszczonym kościele. Po otrzymaniu pieniędzy Amador ma wskazać miejsce, gdzie znajduje się wirus.

Alan Milliken szantażuje prezydenta USA - nie udzieli mu poparcia, jeśli nie pozbędzie się Wayne'a ze swojego otoczenia. CTU organizuje zasadzkę, w której agenci chcą przechwycić wirus.

Ramon zabija Hectora. Sherry znajduje fakty kompromitujące Millikena. Jack i Ramon zastawiają pułapkę na Amadora. Chase z ukrycia obserwuje rozwój wypadków. Jack kontaktuje się z CTU.

Jack i Chase odnajdują Ninę i odlatują z nią do Los Angeles. Chappelle aresztuje Chloe. W systemie komputerowym CTU pojawia się wirus. Jack dowiaduje się od Niny o Marcusie Aversie.

Chappelle sprowadza specjalistę od tortur. Nina po przesłuchaniu trafia do szpitala, z którego ucieka. Próbuje zniszczyć materiały w CTU.Amador przekazuje Alversowi pojemnik z wirusem.

Stephen Saunders odbiera od Amadora i Alversa fiolkę z wirusem. Chce wypróbować go na niewielkim obiekcie. Chappelle grozi Jackowi, że odsunie go od śledztwa, ponieważ zabił ważnego świadka.

Michelle zabija mężczyznę, który chce uciec z kwarantanny. Umiera zarażony Gael. Jack aresztuje Amadora i doprowadza do CTU. Saunders grozi prezydentowi, że zabije połowę Amerykanów...

Saunders zmusza prezydenta do urządzenia konferencji; żąda też śmierci Chappelle'a. Helikopter ostrzeliwuje pokój przesłuchań i Diana zostaje zabita. Michelle poddaje się testom.

Jack kontroluje, aby nikt nie opuszczał budynku CTU. Chloe namierza konto bankowe, którego używał Saunders. Z hotelu znika jeden z gości i jest obawa, że wirus przedostał się na zewnątrz.

Chloe dowiaduje się, że Saunders ma córkę w Santa Barbara. Policja postanawia posłużyć się bardzo do niej podobną Kim. Chase śledzi mężczyznę, który opuścił hotel. Uciekinier jest zakażony.

W CTU Tony przesłuchuje córkę Saundersa. Przekonuje ją, że ojciec jest zagrożeniem dla kraju. Chloe namierza miejsce, gdzie znajduje się Saunders. Prezydent Palmer próbuje uspokoić prasę.

Jack daje rozkaz do szturmu na kryjówkę Saundersa, ale okazuje się, że on opuścił już budynek. Sherry Palmer spotyka się z senatorem Keelerem - przeciwnikiem jej byłego męża.

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.