In 1993, the IRA member Collette is arrested in the London tube after leaving a bomb in the facility. MI-5 Agent Mac offers a deal to Collette to become an informer. She accepts the agreement to protect her son and in return Mac offers a new identity to her after a period working for the MI-5. Soon Mac learns that his superior Kate Fletcher is using Collette to protect her mole inside the Irish organization. Mac tries to find the identity of the informer and protect Collette.