Introduced in the fifth season, Thomas Grace is recruited into Authorized Personnel Only, or APO, a black-ops division of the CIA, in order to replace agent Michael Vaughn after Vaughn is believed to have been killed. Sydney Bristow doesn't particularly connect with Grace for this reason. Grace forms a bond with another recruit, Rachel Gibson, and trains her on how to fight.Not much is known about Grace's background, although some unknown details in his file led some to reconsider welcoming him into APO with open arms. Marshall Flinkman discovers at one point that Grace used to be married. Grace does not take well to Marshall's snooping.