Storylines
Like Sydney, he initially believes SD-6 is a black ops division of the CIA. Because he is not a field agent, he often doctored photographs of himself in exotic locations to fool his mother into thinking he works for a company requiring a lot of travel; he once asked Sydney to take the photographs for said images. When SD-6 is destroyed, Marshall is brought into the CIA for real. Unlike his colleague Marcus Dixon, he adjusts to the change easily and soon becomes a valued member of the team. Starting in the second season, Marshall found a love interest, an NSA agent with similar geekish tendencies, Carrie Bowman. Eventually the two got married and had a child, Mitchell.
During the third season, Marshall experiences great character growth, realizing most people do not understand quantum entanglement, understanding how he appears to others and offering suggestions for operations (which are implemented without comment). At the conclusion of the third season, Marshall is shot and seriously wounded by Lauren Reed. By the start of the fourth season (several months later in the timeframe of the series), he has fully recovered from his injuries and is recruited into the new Authorized Personnel Only (APO) black ops offshoot of the CIA.
Marshall serves as comic relief for the Alias series, but he has some serious moments when he is worried about those he cares about, including Sydney, but mostly revolving around Carrie and Mitchell. Marshall has also discovered how hard it is to lie to those he loves, having to keep his job at APO a secret even from his wife (who appears to no longer be working for the NSA by the fourth season).
Marshall rarely works as a field operative, and then only when his expertise in computers is required. In the fourth season episode "Tuesday", he is forced to go on a solo mission to rescue Sydney, buried alive somewhere in Cuba, after the rest of APO is incapacitated when a biological agent released by an enemy forces a lockdown of headquarters. Marshall, delayed in arriving at HQ due to a problem with his baby, avoids being caught in the lockdown and becomes the only APO agent able to save Sydney. Later in the mission, he accidentally shoots and kills an enemy agent; it is not made clear whether this is the first time he has killed anyone.
The fifth season was largely business as usual for Marshall, although he has also struck up a friendship-cum-mutual admiration society with new APO recruit Rachel Gibson. The series finale saw Marshall return briefly to the field as part of APO's efforts to capture intel on Prophet Five. He was later kidnapped from his home by Kelly Peyton and tortured, resisting admirably until at Sydney's urging he cooperated as part of a plan to rescue him and the also-abducted Rachel. Marshall continued to provide tech support as the various members of APO raced around the world to thwart the endgames of Arvin Sloane and Irina Derevko, including remotely inputting the codes to stand down missiles Irina had targeted on Washington, D.C. and London.
In a flash forward at the very end of the final episode, it is revealed Marshall and Carrie have two more children and Carrie is pregnant with their fourth.
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