When charismatic terrorist Adem Qasim (Elyes Gabel) escapes from MI5 custody during a high profile handover, legendary operative Harry Pearce (Peter Firth), head of counter-terrorism, is blamed. Disgraced and forced to resign, Harry disappears without a trace. With MI5 on its knees in the wake of the Qasim debacle and facing controversial reform, disgraced former agent Will Holloway (Kit Harington) is brought back from Moscow to discover the truth about Harry's disappearance and, in doing so, uncovers a shocking revelation: Harry is still alive, has gone rogue and desperately needs Will's help. Will must decide whether to turn Harry in, or risk everything by trusting the damaged, dangerous master spy who betrayed him once before.