In a "live" broadcast on Halloween night, a BBC team investigate a reported poltergeist in an ordinary London home.

Based on a novel by Colm Tóibín, Eilis Lacey leaves small town Ireland for a better life in New York, arranged by an Irish priest in Brooklyn. Working in a shop she takes a bookeeping course and participates in the Irish community. There she meets an Italian, and falls in love. They marry but she wants to see her mother after the death of her sister in Ireland. Returning home she falls into the life of the small town, meets a local guy, but also a nasty neighbour who knows she was married in the US.

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.