Superintendent Stella Gibson (Gillian Anderson), a Senior Detective from the Met, is seconded to Belfast to supervise a 28-day review into the investigation of the death of Alice Monroe. A.C.C. Jim Burns (John Lynch) recommends Stella view the case as a single entity, rather than as part of a larger pattern. As reporters begin to descend on Gibson, she asks to be introduced to Sergeant James Olson (Ben Peel), after spotting him at an unrelated crime scene, and she tells him her hotel room number.

Gibson is called to the scene of Sarah Kay's murder, a crime for which PC Danielle Ferrington (Niamh McGrady) blames herself. Ferrington admits to Gibson that after unsuccessfully attempting to reach Sarah Kay the evening after the break-in, she and her partner left the premises, likely unknowingly leaving Kay with her attacker. Burns agrees to make Gibson the permanent Senior Investigating Officer (SIO) of the investigation, after she convinces him to admit that the murders are linked. Prof. Reed Smith (Archie Panjabi) arrives at the crime scene, joining the investigating team. DS Olson shows up at the scene of Sarah Kay's murder, asking Gibson why she has been ignoring his messages. Unimpressed by his breaching of the crime scene and of his sending her picture mails earlier, Gibson dismisses him.

Morgan Monroe (Ian McElhinney) coerces Burns into clearing his son's name in relation to killings, whilst DS James Olson is assassinated outside his home.

Gibson builds her team, whose investigation is now known as Operation Musicman. Impressed by Ferrington's honesty at the crime scene, Gibson asks her to be her right-hand man. Gibson is questioned about her one-night stand with James Olson, whom she has recently discovered was married. Gibson begins profiling the killer with her team.

Chief Inspector Matt Eastwood (Stuart Graham)'s investigation into DS Olson's death reveals the involvement of DI Breedlove (Michael McElhatton) with the Monroe family, forcing the latter to take drastic measures that drag Stella deeper into Internal Affairs investigations. An old friend of Prof. Reed Smith, Rose Stagg, tells Gibson that a man named Peter had strangled her to near-unconsciousness while she was at university, nine years ago.

Gibson begins to investigate the botched attack on Annie Brawley (Karen Hassan), bringing her one step closer to the killer, both emotionally and physically. The investigation begins to focus on a single individual. Gibson and Sarah Kay's father mount a televised plea for information pertaining to the murders. Gibson announces the killer will have an opportunity to converse with her privately, if he comes forward.

Ten days after her telephone call with the killer, Superintendent Gibson tries to help Annie Brawley recapture her memories of the attack that left her hospitalised. Rose Stagg (Valene Kane), a friend to Smith, is inadvertently dragged into the case, forcing her to reveal secrets from her past to her husband Tom (Jonjo O'Neill).

Meanwhile, following a break-up with Sally-Ann, Spector returns to Belfast to tie up loose ends, much to the excitement of Katie, and to the dismay of his pregnant wife, who believes he raped the children's babysitter.

Gibson is called in front of the PSNI's Policing Board, headed by Morgan Monroe, to discuss the lack of progress in the case, and Ferrington requests a transfer off of the investigation. DCI Eastwood is assigned as Gibson's Deputy SIO following Brink's (Frank McCusker) reassignment. Gibson declares the investigation will focus on a single individual—Paul Spector—whose partial fingerprint has just been identified on the recovered murder weapon.

Gibson orders extensive surveillance on the suspect and his family. Gibson and Reed Smith share an intimate moment in the jazz bar of Gibson's hotel, but Smith finally chooses not to consummate the sexual liaison. Stella returns alone to her room, where she receives the visit of a distraught Burns who informs her that he leaked information to Morgan Monroe about her investigation, tipping off his son and one-time-suspect Aaron. Burns (a recovering alcoholic who once shared an extramarital affair with Stella and has now relapsed into drinking) aggressively attempts to kiss her, and she fends him off with a palm strike. While she is cleaning his bloody nose in the toilet, Burns mentions that he was the arresting officer in a high-profile raid against a pedophile ring in one of the boys' homes, Gortnacul, where Spector had stayed, and speculates that Spector could have been abused as well.

Gibson's huge surveillance operation continues to pay its dues. DC Martin (Emmett J. Scanlan) and DC McNally (Bronágh Taggart) head a team assigned to install closed circuit cameras and microphones in the suspect's home, a task that has unexpected consequences. The body of a young woman is found apparently fitting the description of Rose Stagg, but she turns out to be a heroin addict who had hanged herself. Gibson seizes the opportunity to bait the serial killer, by convincing young and eager DS Tom Anderson (Colin Morgan) to withhold the fact that the dead woman is not Rose from the press.

Gibson continues to monitor the killer, and Burns visits the disgraced paedophile priest who headed the Gortnacul Boys' Home, Fiachra Jensen (Sean McGinley), who tells Burns that he did not abuse Spector. Ferrington responds to a call about a man matching the description of the suspect who has been taken hostage; the man turns out to be indeed Spector, being held at gunpoint by Jimmy Tyler after a brief scuffle and interrogated about the whereabouts of Jimmy's estranged wife Liz. Jimmy's accomplice shoots Ferrington in her flak jacket and is shot dead by her fellow officer, but Jimmy escapes.

Meanwhile, while Katie works to destroy evidence in Spector's hotel, Hagstrom (Kelly Gough) heads a team to intercept and arrest her, Martin and McNally arrest Sally Ann for perverting the course of justice. Given that the chaos caused by Tyler in Belfast had compromised the entire surveillance on Spector, Gibson is forced to enact the rest of her strategy by having Anderson arrest Spector for the abduction of Rose Stagg.

Gibson views the cybersex video on Katie's phone and asks Anderson to interview Katie based on his superficial physical resemblance with Spector. Several people try to interrogate Spector, who refuses to talk to anyone but Stella. Sally Ann suffers a miscarriage during interrogation, when informed about the crimes Paul is arrested for. Jimmy Tyler manages to locate Liz's women's refuge in Bangor where he assaults her and her fellow residents and nearly shoots them dead.

Spector decides to confess all of his murders to Gibson, but remains tight-lipped about Rose's fate and whereabouts. Their interview is the first chance they have to speak face to face, and he uses his knowledge of Gibson's dream journal to taunt her with his interpretation of her emotional fixation with her deceased father.