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.'

西德妮的母亲原是前苏联间谍，现 在则是某犯罪集团的首脑，一手掌握黑白两道许多人物的丑闻，在枪伤雪梨后以赎罪的心态向中情局投诚，并多次提供可以…

年轻美丽的雪梨.丝斯朵拥有两个截然不同的身份。表面上她是个大学研究所的学生，但实际身份却是SD－6情报员，雪梨以为她是在为国家工作，因为SD－6 …

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.

西德妮-安妮-布里斯托是一个年轻充满活力的大学生，同时，也是神秘组织SD-6(CIA/中央情报局的最高秘密分支机构)的新生代情报员。几年后，当她将自己…

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.

西德妮·布里斯托（Sidney Bristow）是一名大学生，她有一份不同寻常的工作——她是一名中情局（CIA）特工，工作处于保密状态，就连她的朋友们都不知道她的职业。当她的男友向她求婚时，西德妮向他坦白了自己的工作内容，结果她的上司按照规定将其男友杀害。西德妮决定在自己也惨遭毒手之前离开中情局，这时她却发现自己实际上……

五个月后，Sydney辞去了中情局（CIA）的工作，与Dixon、Jack、Vaughn以及Sloane一同加入了一个名为“授权人员专属”（Authorized Personnel Only，简称APO）的特别行动小组。为获得有限豁免权，Sloane选择与APO合作。与此同时，Sydney在维滕贝格（Wittenberg）发现的真相危及了她和Jack的关系，最终，她向Vaughn透露了那个可怕的真相：关于她父亲杀害Irina……

克劳斯在成功抓获《尸》的同伙后返回阳炎宫。 然而，在那里得知的却是缇雅、莫妮卡、安妮特和爱尔娜选拔组的失踪。 选拔组的女孩们在度假时遇见了一位因失忆而来历不明的女孩——安妮特的母亲。 每个人都对重聚感到兴奋，但这却是分裂团队残酷命运的开始。 失踪少女的《秘密的任务》到底是什么？ 世界最强的间谍们、世界最大的欺骗！