The story takes place in Ireland in the 1980s. The internal struggle is in a white-hot state, and the whole country is in a state of extreme tension and tension. Martin (Jim Sturgess) is a 22-year-old ordinary youth. After receiving training from the British police, he was sent to the IRA alone and became an undercover. Martin's mission is to disintegrate the Republican Army from within and prevent the terrorist activities carried out by the Republican Army. Every day, Martin is accompanied by death, and as long as there is a careless danger of exposing his identity. In order to gain the trust of the enemy, Martin has experienced unimaginable torture and suffering, and finally successfully broke into the inside. But the good times didn't last long. An accident exposed Martin's undercover identity, and he had to embark on a journey of escape.