In 2003, senior CIA official Fitzroy visits a prisoner. Eight years earlier the prisoner was a minor convicted of killing his abusive father to protect his brother. Fitzroy offers the man his freedom in exchange for working as an assassin in the CIA's Sierra program. In 2021, on a mission in Bangkok, the prisoner Sierra Six, is working with fellow CIA agent Dani Miranda to assassinate a target suspected of selling off national security secrets. He is unable to do so stealthily without harming civilians and attacks the target directly, mortally wounding him. Before dying, the target reveals that he worked in the Sierra program as Sierra Four, and hands Six an encrypted drive detailing the corruption of CIA official Denny Carmichael, who is the lead agent on the assassination mission. Carmichael is elusive about the true purpose of the mission and the contents of the drive when confronted by Six.