The final episode commences with a flashback, showcasing Sylvia, Ember's mother, tenderly cleaning a severe cut on Mack's hand. She chides him for missing Ember's 17th birthday, while he excuses himself with a lie, claiming he hurt himself slicing a bread bap during a night shift.
Outside the boat house, Amy and Caitlin join Mack and Joe. Amy inquiries about his injured hand, revealing that it was actually her who slashed him during their heated argument. Caitlin's horrified realization dawns as she remembers the blood splattered across the floor.
In the present, Ember faces interrogation following her arrest. DI Morgan presents the bone fragment found in the locker, confirming it belongs to Amy Knightly. He implores Ember to aid in locating the remaining remains, as they were not discovered alongside the fragment in the boat hut. Ember, however, insists she knows nothing and cannot assist.
Released on bail, Ember returns home on the anniversary of Mack's passing. They gather for a special breakfast to commemorate him. Ember attempts to broach the subject of the evidence found, but Hannah refuses to listen. She desires to scatter her father's ashes at the jetty, yet Ember insists they cannot due to the ongoing police investigation.
A flashback transports us to a conversation between Amy and Caitlin, where Amy reveals her intention to have her first sexual encounter with Mack and urges Caitlin to do the same with Joe, thus losing their virginity together. Though hesitant, Caitlin agrees to please Amy. The girls share a kiss, unseen by Caitlin's father but witnessed by Joan, Caitlin's mother, who conceals it from her husband.
On the anniversary of Mack's death, Ember, Sylvia, Arj, and Hannah raise a toast in his honor. When Hannah prompts her mother to speak about Mack, Ember struggles for words. Hannah's frustration boils over, and Ember reveals the discovery of Amy's bone fragment at the boathouse. Enraged, Hannah rushes out, spilling Mack's ashes on the floor. Ember frantically gathers them while Hannah demands proof of her father's innocence.
Ember once again visits Joan at the care home, seeking answers about Kitty. But the doctors have administered a sedative after Joan became agitated during Ember's previous visit. Caitlin's father intercedes, explaining that "Kitty" is a nickname for Caitlin and requests Ember to leave.
Ember eventually tracks down the now adult Caitlin, who works as a university lecturer. Initially, Caitlin puts up a facade of ignorance, pretending not to recognize the name Amy Knightly. However, when Ember presents the photograph she found in Mack's study, Caitlin's demeanor softens. She tells Ember that Amy probably led a fulfilling life after fleeing 17 years ago, but Caitlin is devastated when Ember breaks the news that Amy is deceased.
Ember inquisitively asks Caitlin why she lied to the police about being with Amy on the night of her disappearance. Caitlin reveals that her mother cautioned her to maintain distance, fearing that if the police discovered her romantic connection to Amy, they might suspect her involvement in the crime. Ember queries Caitlin about who took the photograph, and Caitlin reveals it was Joe, Mack's brother, leaving Ember stunned, unaware that her late husband even had a sibling.
Ember informs Hitch about Joe, but Hitch's concerns lie elsewhere, worried about job security as they spend time together while she is suspended. Ember begs for his assistance in visiting Joe, who is incarcerated. Hitch struggles to comprehend how Joe could have murdered Riz, but Ember reveals that Joe has been granted day releases, including the day Riz was killed.
During their visit to the prison, Joe claims ignorance, saying he hasn't seen Mack in years and denying knowledge of the girl in the photograph with Amy and Mack. A flashback reveals the truth; Joe does know, as he captured the image on the fateful night Amy vanished. Although Ember is convinced Joe is Amy's killer, Joe is astonished when she informs him of Amy's death and the discovery of her remains at the boat hut, proving his innocence.
In the flashback, Joe and Caitlin are seen sleeping together, paralleling Amy and Mack's arrangement. However, when Joe confesses that it's not the first time his brother has slept with Amy, Caitlin realizes her best friend deceived her to sleep with Joe. Distraught, she rushes off, and Amy pursues her. The girls engage in a heated argument, and Caitlin realizes they are friends, uncovering Joe's scheme of blackmailing his brother and threatening to report him for sleeping with a minor unless they found him a willing partner. Heartbroken by the revelation, Caitlin rushes away, telling Amy she loves her before departing.
Back at the boathouse, Mack ends his relationship with Amy, confessing that he is leaving, that he doesn't love her, and that he feels guilty for what they did to Caitlin. He advises Amy to pursue Caitlin, revealing that she is in love with her. Amy begs Mack not to leave, but Joe steals his car keys and flees, leaving the couple in turmoil.
In the present moment, Ember sits in her familiar bedroom at her mother's, reliving the night of her birthday. Mack's voice echoes in her memory as he sings a song he composed for her. He apologizes for having to leave for work, a facade she now knows was merely a ruse to meet Amy and Caitlin for the last time, under Joe's coercion. Before leaving, he offers her some drugs, but Ember recalls how they made her ill.
She turns to her mother, Sylvia, inquiring once again about the night Amy disappeared. Sylvia recounts that Amy left the party early, and by the time she returned home, Ember and Mack were already present. Ember presses on the subject of drugs, but Sylvia dismisses it as trivial, disclosing that Mack didn't stay the night as they had a task to accomplish.
Ember probes deeper, asking who "they" are, and a realization dawns on her. She rushes to the boathouse, where she finds Arj drowning his sorrows at the jetty. She confronts him, revealing that she knows he was with Mack on the fateful night. Arj, inebriated, muses about a potential relationship between them, but Ember ignores his drunken ramblings.
Suddenly, the rumble of a boat in the dock jolts Ember's memory. She realizes it's the same noise she heard in the background of the audio file where Riz's murder was recorded. With a chill in her spine, Ember realizes Arj is the one who killed Riz. She attempts to flee, but Arj grabs her, confessing his crime. He claims he never intended to harm her, but she came seeking him. He explains that the bone fragment was in the boat hut because he, upon hearing the builders tearing up the floor, broke in, dug up the body, and set fire to the boathouse to obliterate any trace of his involvement.
Ember confronted Arj, urging him to have left Amy's body to be discovered, allowing Mack to bear the blame for his actions, and letting Riz live. But Arj dropped a bombshell, revealing that he hadn't just killed Riz; he had also killed Amy—it wasn't Mack. Ember was stunned as he tried to make her listen, recounting that on the night of her birthday, they were at the pub, and he felt sorry for her because Mack wasn't there. He knew where Mack really was—with Amy—and Ember deserved better. He reminded her of that night, when she was doling out pills and everyone was having a blast, but then he realized Mack had been leaving him messages, asking for a ride from the boathouse.
Flashbacks revealed a figure driving down a dimly lit road, picking up Mack. As they sped off, they struck Amy, who was standing by the roadside. Mack lied, claiming it was just a log. In the present, Arj confessed that he didn't recall driving that night, only waking up in bed with Mack shaking him awake, revealing that he'd hit Amy and they had to dispose of the body. She'd fallen down an embankment, her head striking a rock. They considered dumping her in the lake, but Mack suggested the basement of the boathouse would be kinder.
As Arj recounted that fateful night, Ember's memories clicked into place. She realized it wasn't Arj driving that night; it was her. She had killed Amy, and Mack had covered for her to protect her.
Arj confessed that Amy's death had ruined his life, and they argued heatedly when Ember insisted they must go to the police and tell the truth. But knowing he'd face jail for killing Riz, Arj snapped, strangling Ember in a fit of rage, mirroring the way he'd killed Riz.
Ember arrives at the end of the jetty, her fate hanging in the balance. Arj, his face twisted in a manic rage, douses her, the jetty, and himself in petrol. Ember pleads for mercy, but Arj shouts that he did it all for her, claiming that love drives people to strange lengths. She retorts that this isn't love, but her words are drowned out as he ignites a blaze, engulfing them all in flames. With no other option, Ember plunges into the water, desperate to escape the inferno.
Later, Ember sits in a police station, photographs documenting her bruised neck taken. The following day, she rides in silence with Caitlin, who is angry that Ember didn't reveal Malachy was her husband during their last encounter. Ember apologizes, and Caitlin warns that Amy was never safe with any man. However, Ember interrupts, confessing that she was the one behind the wheel, not Arj. She reveals her ignorance of being Amy's killer and declares her intention to surrender. Caitlin questions if truth and justice align, implying Ember shouldn't confess after so long.
At the police station, Ember is grilled by DI Morgan, who demands she reveal all. Yet, Ember shifts the blame onto Arj, refusing to confess the truth. We then witness Caitlin visiting her mother in a care home, her parents overjoyed. Meanwhile, Ember, accompanied by Miranda and her newborn, declares her intention to compile a list of men she had sex with underage, vowing to imprison them and Liam. She aims to create a safer world for her daughter by putting these men behind bars.
The episode concludes with Ember, Hannah, and Sylvia at the lake, scattering Mack's ashes. Ember acknowledges Mack's mistakes but insists they won't stop loving him. Sylvia apologizes for not being the best mother and hopes it's not too late to make amends. They embrace, surrounded by the serene lake and the memory of Mack.
