Published in 2016, The Trespasser is a crime fiction novel by Tana French. Set in contemporary Dublin, Ireland, the story follows Detective Antoinette Conway, the lone female member of the famous Murder Squad, whose routine domestic murder case turns out to be anything but. The Trespasser is the sixth novel in French’s Dublin Murder Squad series. Called the “First Lady of Irish Crime” by The Independent, French was born in the United States but resides in Ireland. Her debut novel, In the Woods, won the Barry, Edgar, Macavity, and Anthony awards for best first novel.

Rookie detective Conway and her partner, Steve Moran, are part of the elite Dublin Murder Squad. Conway and Steve have just been assigned their first big case but are annoyed to be saddled with senior detective Breslin; he is only supposed to assist but wants to oversee their every move. The two detectives find an attractive young woman named Aislinn Gwendolyn Murray dead on her living room floor. A punch to the jaw caused Aislinn to fall against the fireplace and crack her skull. Conway can’t shake the feeling that she’s seen this woman somewhere before.

On the night of her death, Aislinn was expecting company; she had set the table for a romantic dinner. Though the detectives assume this was a date gone wrong, the murderer took the time to shut off the cooking stove and avoid tripping the smoke alarm; such an action would indicate premeditation. After scanning texts on Aislinn’s phone, the detectives discover that she was supposed to have dinner with a new boyfriend named Rory Fallon. She texted her girlfriend, Lucy Riordan, to tell her how much she was looking forward to the date.

When Conway and Steve question Lucy, she’s evasive about the date and the boyfriend. The detectives suspect she knows more than she’s telling. She does say that Aislinn devoted her life to taking care of her mother after her father went missing years earlier. Until her mother died, Aislinn had been overweight and frumpy. Afterward, she transformed herself into a trophy girlfriend. She may have had a secret boyfriend who is married.

The detectives go back to the station to interview Rory. Breslin wants to handle the interrogation with Conway. The senior detective takes an aggressive approach, and Rory fears that he might be considered a suspect. He says when he arrived for his date, Aislinn never answered the door, so he left. Breslin wants to railroad Rory into a confession, but Conway stalls him. Later, Conway and Steve discuss the possibility that Breslin is somehow involved. Aislinn’s secret boyfriend might have mob connections, and Breslin may be taking bribes from gangsters. Conway and Steve also speculate that Breslin might have called in the anonymous tip about the murder.

When Conway holds her first case meeting, she’s acutely aware that she has many enemies in the department. As the only female and biracial detective, she feels hostility from her coworkers. Someone has stolen one of her witness statements and urinated in her locker. She believes that only Steve is on her side. Conway and Steve search Aislinn’s apartment but find nothing. They check the local bars, and one bartender recalls seeing Aislinn with an older man. Conway finally remembers where she saw Aislinn before: Two years earlier, while Conway was still working Missing Persons, Aislinn had come in pleading for fresh news about her father’s disappearance. Conway was irritated by Aislinn’s weepy approach and sent her away. Conway’s own father abandoned her mother before she was born, yet she didn’t let it wreck her life. Steve speculates that Aislinn may have conducted her own investigation on the side because the police wouldn’t help her.

The detectives try to keep their theories to themselves because they don’t trust Breslin. Conway finds fresh evidence that someone is sabotaging her investigation, and she concludes that the rest of the department wants to drive her out. She contemplates resigning after the case is over. The investigation becomes far more complicated than expected when Conway goes back to interview Lucy. She reveals that Aislinn radically changed her appearance because she wanted to target the detective assigned to her father’s Missing Persons case—McCann. She didn’t want him to recognize her. Aislinn, posing as a badge bunny, eventually gets McCann to talk about the details of the case. Aislinn confides to Lucy that she became enraged when she learned that McCann failed to disclose that her father ran off with another woman. He had no right to withhold that information and ruin her life.

Aislinn now intends to ruin McCann’s life. She concocts a scheme to seduce him, get him to leave his wife, and then dump him. Her plan proceeds smoothly until she falls in love with Rory. The night Rory is supposed to come over for dinner, McCann shows up unexpectedly. Aislinn abruptly tells McCann that it’s over but won’t say why. She wants him to go on wondering for the rest of his life, just as McCann left her wondering what became of her father. McCann overreacts and punches Aislinn. She dies from her injuries. Later, McCann asks for Breslin’s help to cover up what he says was an accident.

Conway and Steve interrogate McCann. He never knew that Aislinn was the little girl whose father disappeared and that she wanted revenge against him. He’s on the point of cracking when Breslin stops the interrogation and bullies the two detectives into suppressing the interview tape. Conway and Steve go to see their chief. They tell O’Kelly everything they’ve discovered. He believes them and is angry that Breslin lied about McCann’s involvement in the crime. McCann’s and Breslin’s careers are over. Conway and Steve are vindicated in their chief’s eyes and in the eyes of their colleagues. Conway is relieved to know that no one was out to destroy her career. She still has a future on the Murder Squad.