Sydney Bristow was recruited as an undergrad to work for SD-6, which she thought was a government agency. However, when her fianc is killed by SD-6 agents, she finds out that SD-6 is actually a counter-government agency. Sydney approaches the CIA and becomes a double agent, which means that she goes on SD-6 missions, but passes the information about her missions to her CIA handler, Vaughn....Sydney Bristow是政府秘密分支机构SD-6的情报员，但是她的男友带来了杀身之祸，雪梨在向SD-6的长官求证后，发现男友正是死在SD-6的手下，同时她发现SD-6实际上是反政府的机构。Sydney加入了CIA成为一个双面间谍，也就是说她在完成SD-6分派的任务的同时，将任务的细节情报汇报给她CIA的上司，Vaughn....