For seven years, Sydney Bristow believed she was working for a covert branch of the CIA, named SD-6. However, when she tells her fiance of her job, Arvin Sloane, head of SD-6 and an old family friend, has him murdered. With this life-shattering event, Sydney learns not only that SD-6 is a mercenary group of terrorists, but that her father, too, works for SD-6. Sydney then works with the CIA as a double agent, trying to bring down SD-6 with her father, a second double. This is only the beginning of a path of betrayal, murder, lies, and disbelief.