Thomas "Tom" Grace was an agent working for APO. He was recruited by Jack Bristow after the supposed death of Michael Vaughn. He eventually sacrificed his life to save many people when Julian Sark placed a bomb under the APO headquarters.
Before Season 5
Tom had a wife named Amanda Grace who was killed.
The character was introduced in the second episode of the fifth season. Thomas Grace is recruited into Authorized Personnel Only, a black-ops division of the CIA, in order to replace agent Michael Vaughn after Vaughn is supposedly killed (he also replaces Eric Weiss after Weiss transfers out of APO). This led to friction between Grace and Vaughn's partner and fiancée, Sydney. Grace also bonded with another new APO recruit, Rachel Gibson, and began training the novice agent to fight.
Not much is known about the character's background, although his file, details unknown, caused many a person to halt and reconsider welcoming him into APO with open arms. In The Horizon, Marshall Flinkman discovers that Grace used to be married, though he initially believes it is a remnant from an undercover assignment. Grace reacts with anger at Marshall's nosiness.
In S.O.S, Grace takes part in an APO mission to infiltrate CIA headquarters in Langley, Virginia in order to locate Sydney, who has been kidnapped by Prophet Five. During the mission, he takes advantage of the opportunity to obtain information from the Witness Protection Program regarding the man who, it is revealed, murdered Grace's wife Amanda. Later, Grace tracks the man down, but instead of killing him, asks to be put in touch with someone called "The Cardinal." His "off-book" mission does not go unnoticed, as Rachel discovers Grace's investigation. Grace, in response, lies to her and says he intended to kill the man, but was unable to do so once he saw that he had started a family of his own.
In No Hard Feelings, Rachel discovers the truth: The Cardinal is the man who ordered the death of Grace's wife and the man he had pursued was the hit man assigned to do the job. The hit man agrees to tell Grace why his wife was targeted, in return for his vehicle which had been impounded by the government. Upon returning the vehicle to the hit man, he is told that Amanda was simply "in the wrong place at the wrong time" and that the intent was actually to kill Grace. The hit man then drives off in his vehicle, only to die when Grace detonates a bomb inside. The identity of "The Cardinal" and why he had targeted Grace for assassination is never revealed.
In the penultimate episode of Alias, Grace is sent to defuse a bomb that Sark had placed onto a subway to destroy APO headquarters. Unable to defuse the bomb, he repeatedly sprays liquid nitrogen on it, which slows down the mechanism for a short period of time but does not disable it. He is instructed to leave when the timer gets to 60 seconds, but ultimately stays with the bomb beyond the one-minute threshold in order to keep it from going off. His last words are to Rachel Gibson, telling her that he wished they had more time because he would have asked her out. Starting to cry, Rachel responds that she would have said yes just as the bomb detonates, killing Grace. (Reprisal).