The following version of this book was used to create this study guide: Berry, Flynn. Northern Spy. Viking, 2021. Kindle Edition.

Tessa is a reporter who lives on the outskirts of Belfast, Northern Ireland. She works for the BBC where she produces a radio show. She is also a single mother who shares custody of her infant son, Finn, with her ex-husband, Tom. While the strife that existed in Northern Ireland is officially over, the IRA still lurks in the background. Tessa is not an IRA supporter and does not think that anyone in her family is until she sees her sister on television about to take part in an IRA sponsored robbery. Tessa cannot believe that her sister participated voluntarily. She believes that she was kidnapped and forced to take part in it. However, the police believe that her sister is a member of the organization.

After the robbery, her sister goes into hiding for a period of time. Then she resurfaces and contacts Tessa and their mother. Tessa discovers that her sister is truly a member of the IRA, but she is now working as an informer. She wants Tessa to take over her duties of contacting Eamonn, her handler who works for MI5, because she cannot do so anymore. At first, Tessa refuses. Later, after thinking about Finn and his future, she decides to meet Eamonn. Tessa meets him on several occasions and passes information from her sister, Marian, to him. During this time, she starts to feel a physical attraction to him.

Things become more complicated for Tessa when she goes to her cousin’s wedding. There she meets Seamus, Damian, and Niall, who are members of Marian’s IRA unit. A man named Cillian Burke is also there. He is a member of the IRA who was put on trial but released because MI5 would not provide the necessary evidence to convict him. Seamus thinks that Tessa would be a good addition to the IRA. Tessa is reluctant to join, but she does so when Marian says things will not look good for her if Tessa does not join. Eamonn agrees. Tessa says yes.

At first, the group has her do small surveillance jobs. Then, they have her participate in an assassination attempt with Damian. Damian misses the target but does not get in trouble for doing so.

A cease-fire is announced soon afterward. Tessa and Marian celebrate. One night, masked members of the IRA come to Tessa‘s house to take her away. She gets them to agree that she can take Finn to her neighbor’s house. While there, she tells the neighbor to call DI Fenton, the detective who is investigating the robbery.

Tessa returns to her home and lets the kidnappers take her away. When she arrives at the destination, she recognizes the guard and talks to him. She is left in a room alone until Marian, who has also been kidnapped, arrives. The next morning Seamus comes to talk to them. Marian tries to take all the blame, but Tessa speaks up as well. When it becomes obvious that Seamus thinks they are guilty of being informers, Marian kills him. The two sisters flee and find a sympathetic woman who allows them to use the phone. Tessa calls DI Fenton. Arrangements are made so that they can go to the hospital across the border.

The authorities give them immunity, and the two women enter a protection program. A newspaper report says they are dead and they receive new identities. They move to a location outside of Dublin along with Finn. Their mother later joins them in a neighboring location. They learn that Burke was also an informer and that he had leaked the information that Seamus had accused Marian and Tessa of leaking. The ordeal allows Tessa to find the peace that eluded her in the past.