Set in Belfast in 1980 where the Irish Republican Army is waging an all-out insurgent war against the British occupation, Martin McGartland is a young small-time Belfast hustler recruited by a wily British intelligence agent to infiltrate the IRA. Martin quickly uses his natural instincts as a con man to get inside the IRA and starts feeding information to the British, saving hundreds of lives in the process. But with each successful mission, Martin becomes increasingly in danger of being caught and exposed. The IRA is ruthless, and once an infiltrator has been exposed he becomes a "dead man walking." As Martin becomes consumed by his double life, and his British handlers threaten to sell him out if he refuses to continue his counter-intelligence work, he is plunged into a dangerous game in which the stakes could not be higher.