John Porter (Richard Armitage) resigned from the SAS following the aftermath of a rescue mission in Iraq on the eve of the invasion in 2003, where two fellow soldiers were killed, apparently by a thirteen-year-old boy Porter failed to kill, leaving him responsible. Seven years later, Porter is "reactivated" following the kidnap of journalist Katie Dartmouth (Orla Brady) by the same terrorists he fought seven years previous, including the same boy, As'ad. Porter succeeds in rescuing Dartmouth and joins Section 20, headed by Hugh Collinson (Andrew Lincoln), who was also present during the fateful mission. By the end of the series, it is revealed that Collinson was the one who killed the soldiers, and covered this up. Section 20 officer Layla Thompson (Jodhi May) discovers this, and blackmails Collinson into saving Porter's life in Afghanistan. There, Porter learns that during the mission, Collinson panicked and mistook the soldiers for terrorists. Taliban members raid their location, and Collinson is mortally wounded while aiding Porter's escape.