Alias began airing on the ABC Network in the United States on September 30, 2001. Created by J.J. Abrams (Felicity) the show combined elements of drama, action, espionage and thriller. It starred Jennifer Garner as Sydney Bristow. When the show began Sydney was a graduate student who believed she was working for a black ops division of the C.I.A. called SD-6. Working there with her was Director Arvin Sloane (Ron Rifkin), her partner Marcus Dixon (Carl Lumbly) and op tech Marshall Flinkman (Kevin Weisman). When Sydney's boyfriend Danny (Edward Atherton) proposes to her, she can't help lying to him anymore and informs him of her double life as a spy. The one rule at SD-6, however, is that you do not tell anyone what you do. Drastic action is taken and Sloane has Danny killed. Several months later Sydney finds out that SD-6 is actually a branch of the Alliance of Twelve, a highly dangerous alliance of criminals from around the world that tells most of their agents that they're working for black ops divisions of their government. Sydney is told this information from a very unlikely source - her father Jack Bristow (Victor Garber), whom she previously believed exported airplane parts. With this new knowledge and knowing that Sloane wants her to return to work as soon as possible and is losing faith in her, Sydney solidifes her position at SD-6 while secretly going to the real C.I.A. and offering her services as a double agent. Her handler there is Michael Vaughn (Michael Vartan), he was sometimes aided by Eric Weiss (Greg Grunberg). Sydney is told there is another double agent inside SD-6 and she finds out that her father is that agent when he comes to confirm her status as a double.

Sydney is faced with the arduous task of trying to destroy the Alliance and also is left questioning who exactly at SD-6 she can trust. Matters are made more complicated by Sloane's obsession with finding artifacts from Milo Rambaldi, a 15th century inventor and prophet. Burned for heresy, Rambaldi fortold many events and even managed to write code for a cell phone several hundred years before they were invented. Rambaldi also fortold that a woman bearing a great likeness to Sydney would bring about the world's destruction. Sloane sends his agents all over the world aiming to uncover new Rambaldi artifacts.

As Sydney gets deeper and deeper undercover, she and Vaughn become closer and eventually fall in love. She soon discovers that her mother, who she believed died in a car accident, was alive and had been a KGB agent assigned to marry a U.S. government agent - Jack. Irina Derevko (Lena Olin), Sydney's mother, eventually turned up in the U.S. and turned herself in. She began helping the C.I.A. but many around her still questioned her motives. Irina worked with a young man named Julian Sark (David Anders), whose loyalties were known to be quite flexible. Several years later Sydney discovered that as a result of a tryst between Sloane and Irina, she had a younger sister named Nadia Santos (Mia Maestro). Nadia had been raised in Argentina and eventually joined Argentine Intelligence. They were all eventually joined in their work by Rachel Gibson (Rachel Nichols), Thomas Grace (Balthazar Getty) and Renee Rienne (Elodie Bouchez). All the while, Sydney had to balance her professional life with her private life, risking her friends Will Tippin (Bradley Cooper) and Francie Calfo (Merrin Dungey).

Alias finished its five season run on May 22, 2006 after 105 episodes. The show was produced by John Eisendrath, Alex Kurtzman, Roberto Orci, Jeff Pinkner, Jesse Alexander and Ken Olin. In the final season series star Jennifer Garner was given a producer credit. She also driected an episode in the 4th season. Michael Giacchino composed the music for the show throughout its entire run. J.J. Abrams composed the show's theme music.

Alias was known for having many notable guest stars throughout its run. These include Faye Dunaway, Quentin Tarantino, Ethan Hawke, Christian Slater, Isabella Rossellini, Sonia Braga, Terry O'Quinn, Angela Bassett, Vivica A. Fox, Roger Moore, Djimon Hounsou, Peggy Lipton and Ricky Gervais.