这一季的故事发生在上一季的3年后。这一季里，托尼（卡洛斯·博纳德 Carlos Bernard 饰）和米歇尔（蕾可·艾尔丝沃斯 Reiko Aylesworth 饰）结婚了，并且成为了CTU的主管，而杰克（基弗·萨瑟兰 Kiefer Sutherland 饰）的女儿金姆也加入CTU成为了一 名情报分析员。剧情由一具被扔在洛杉矶卫生中心外并感染了致命病毒的尸体开始。一个墨西哥毒枭为了让美国政府释放自己的哥哥，以致命病毒为恐吓。杰克也因此又踏上了反恐调查之路。而这一季里面，杰克的调查并不顺利，为了打入毒贩内部，杰克不惜染上毒瘾，这导致他在调查中困难重重。而白宫方面，获得连任的鲍尔默总统（丹尼斯·海斯伯特 Dennis Haysbert 饰）康复后身边多出了一位私人医生女友，而总统的弟弟韦恩成为了他的得力助手，在连任压力面前，又要化解这场生化危机。总统还能像前几次那样给予杰克强有力的支持吗？
After reconnecting with her mothers side of the family, Billie Carter (Jessica Mauboy) returns to the city finally knowing that she is Jack Nortons secret daughter. Which means shes now a millionaire. Billie throws herself into her glamorous new life, and just by being Billie, she changes everyone elses world for the better. But theres one thing missing. For all the fairy dust Billie leaves in her wake, she keeps her own heart firmly guarded against falling in love. Surely there must be someone out there for her? But as her career takes off, tragedy threatens. Will Billie realise her dreams, only to lose whats most important to her?
Six people bound together by a traumatic experience decide to round off a year of group therapy in style. They join each other once more, traveling up to the Scottish Lowlands for a reunion weekend in a 1970s holiday let. Soon a number of mysterious and horrifying events start to expose the cracks in their relationships, and one by one they discover that trust runs but skin-deep. A night of closure turns out to be far more final than they could have anticipated.