The following version of the book was used to create this study guide: French, Tana. The Trespasser. Viking, an imprint of Penguin Random House LLC, 2016.

The novel starts with Detective Antionette Conway and Detective Steve Moran going to investigate a murder. They find the house pristine, with no fingerprints or signs of a break-in. Aislinn Murray, the victim, had the table set for two and was clearly in the middle of cooking. She looks like she has been punched in the head, and maybe hit her head on the fireplace, causing her to have a concussion and die. Aislinn is blonde, skinny, and has very common, generic looks. Despite Aislinn’s generic looks, when Antionette goes to examine the body, she feels she recognizes her, although she cannot remember from where.

Conway and Moran go to interview Aislinn’s best friend, Lucy Riordan. Lucy tells them that she was worried about Aislinn, but at first she does not explain why. Aislinn had a new boyfriend whom she really liked and she cannot imagine that he would have murdered her. At the very end of the interview, Lucy says that Aislinn might have had a different, secret boyfriend.

Conway and Moran are being put on the case with a more experienced Detective, Detective Breslin. When Conway and Moran get back to the station, in Dublin Castle, Breslin is there, waiting to interview Rory Fallon, Aislinn’s current boyfriend and the one who was due over at her house on the night that the murder took place. Rory tells the detectives that Aislinn never answered the door. He is a jumpy and skittish person, but does not look like the type of person that would murder anyone. Breslin and Conway let him go, but say they will bring him back.

Detective Breslin watches closely over Conway and Moran and tries to convince them that the murderer is Rory. Conway and Moran are not convinced. Later, Conway finds Breslin talking to his partner, Detective John McCann in the hallway. She only overhears the end of their conversation, but they are talking about getting rid of some girl. Conway thinks that it is her.

As they are finding out more information about Aislinn, Conway realizes how she knows her. Conway met her when she worked for the Missing Persons unit. Aislinn came in trying to find information about her father. Conway did not know much about the case and passed her off to someone else. Conway and Moran pull up those files and realize that her dad went missing years ago. The detectives eventually found Aislinn's dad in England with a new wife, but the detectives did not tell the family. They also find out that Detective McCann, Breslin’s partner and the one who he was whispering to in the hallway, worked this case.

In Conway’s personal life, she meets with a friend who works in undercover. Conway and Moran have hypothesized that Aislinn tried very hard to find her father and got herself into some gang mess. Her friend knows all the gangs, and when Conway brings this up, he almost laughs at her for thinking it. Moran holds tight to this theory, though. Conway and Moran have a fight in front of all the other detectives. Conway realizes that Breslin might have been right the whole time and that it is very possible that Rory Fallon is the murderer.

Conway and Breslin bring Rory back in to interrogate him. They had figured out that he was not entirely truthful last time. When he had told them that he went to the local shop to buy flowers, they realize that there was a lot of unaccounted for time. After some pressing, Rory admits that he had been stalking her. They let him go, even though Breslin thought they were close to getting a confession from him.

Even though everything points to Rory, Conway is still thinking about what Lucy said about the secret boyfriend. After talking with Moran and reconciling as partners after their fight, they decide to pursue this theory and see what they can find. They talk to Rory together and realize that he could not have been the murderer.

Together, after talking with Rory and Lucy, Moran and Conway figure out that the person who Aislinn was secretly dating was Detective McCann. Lucy tells Conway that Aislinn had tracked down McCann because she thought she might know something about her father, and he wanted him to feel the pain she felt when she was a kid and the case was never resolved.

Conway and Moran try to get a statement out of McCann. Unfortunately, McCann knows all their tricks, and even though he admitted to having an affair with Aislinn, he stops answering their questions when they try to pursue the topic further. Breslin finds them interrogating him and tells them to stop. Conway and Moran are still convinced it was McCann though, so they escalate the case to the chief. O’Kelly calls all three of them into his office and gets a confession out of McCann. He accidentally murdered Aislinn when he found out that she was cheating on him with Rory.