Scott’s life is in freefall: his girlfriend has just left him, he’s dropped out of studying medicine and he’s desperate for cash to help save his brother. Out of the blue his terminally ill neighbour comes to him for a favour – to help him commit suicide. Scott reluctantly agrees, and in the process, he and his two friends decide to set up their own illicit assisted-suicide bus...
位于极地的北极生物系统研究基地爆发神秘病毒，造成三人感染、两人死亡的惨剧。为了查明病毒的成因，防止其他一百多名来自各国的科学家遇险，美国疾病防疫控制中心在军方的要求下，派出了艾伦·法拉格特（比利·坎贝尔 Billy Campbell 饰）、茱莉亚·沃克（凯拉·扎戈斯基 Kyra Zagorsky 饰）、莎拉·乔丹（乔丹·海耶斯 Jordan Hayes 饰）与多琳·博伊尔（凯瑟琳·勒米厄 Catherine Lemieux 饰）等四名精英飞往北极。三人中的幸存者彼得（尼尔·纳皮尔 Neil Napier 饰）是艾伦的哥哥，当年因一桩荒唐事导致弟弟和妻子茱莉亚离婚。一行人虽然受到畑博士（真田广之 饰）的热情接待，但基地内似乎隐藏着不可告人的秘密。
表面上，雪莉（珍妮弗·加纳 Jennifer Garner 饰）是一名普通的大学研究生，实际上，她已为名为SD6的秘密情报组织工作多年。在同男友结婚之际，雪莉决定向其公开自己的真实身份，没想到男友却因此被组织的安全人员暗杀。愤怒中，雪莉选择了“罢工”，之后亦遭到了抹杀。
危机之中，雪莉知道了自己的父亲杰克（维克托·加柏 Victor Garber 饰）也是和自己一样的秘密特工，并且，在背后扶持SD6的并非国家组织，而是拥有强大武力的反动分子。于私于公，雪莉和SD6都有着不共戴天之仇，她回到了SD6，但此时，雪莉的身份已升级成了为CIA工作的双重间谍，在危险和考验之中，雪莉的任务只有一个，那就是彻底铲除SD6。
Having woken up in Hong Kong, with no memory for the last two years (!), Sydney returns to Los Angeles to try to piece together the last two years of her life and discovers the fates of her friends and family: Vaughn has married thinking that Sydney was long dead; Marshall is having a baby with CIA tech expert Carrie Bowman; and Dixon is now the new division head of the L.A. office branch of the CIA. Sydney also learns the shocking fates of Jack and Sloane; the former is in CIA custody for communicating with Irina whom still is in hiding, while the latter is living in Switzerland under house arrest in exchange for co-operation with the CIA. Sydney is granted temporary CIA clearance on a case that may help her regain her memory and she meets a new enemy group called The Covenant.
Sydney finds herself face-to-face with her long-lost mother, Irina Derevko, whose intentions for her daughter are made chillingly clear. However, Sydney manages to escape and rejoin Jack and Will in their escape from Taiwan. After arriving back in Los Angeles, Sydney continues her search for Vaughn, not knowing if he is dead or alive after being swept away by a wall of water in Taipei. Will's life is forever changed when he learns the truth about SD-6 and Sydney's life as a double agent. Also, Dixon must decide whether or not to inform Sloane of Sydney's suspicious activities. But Jack intervenes and covers for Sydney who later finds Vaughn who helps her try to track down her mother Irina who's searing for a Rimbaldi artifact called 'the Bible.'
Sidney Bristow is a college student who has an unusual job – she is a spy working for the CIA under secret conditions where not even her friends knows what she does. When her boyfriend proposes Sydney tells him what she does for a living and her employers kill him in line with procedures. Deciding to leave the agency before they kill her too Sydney discovers that she has actually been working for SD6, an agency that is actually a shadowy criminal organisation world-wide. Sydney joins the CIA and works as a double agent to help CIA Agent Vaughn to bring down SD6. She finds that her father is the other double agent within SD6 and the pair work together. While following her missions to uncover the work of the prophetic Rambaldi she must feed information back to the CIA without blowing her cover.
