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。
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.'
