As the curtain falls on Shetland's final episode, Annie Bett (portrayed by Sarah MacGillivray) imparts wisdom to Noah Bett (Jacob Ferguson) about the intricacies of prime numbers from the confines of their car. Noah's gaze wanders to a dinosaur toy before inquiring about their impending return home. Annie reassures him with a promise of "soon." Just moments later, Anton Bergen (Leeroy Boone) inserts himself into their conversation, disclosing that Stefan is aware of their efforts to assist Astrid.Annie, however, has a plan; she knows a hideout where they can lay low until Stefan's fury subsides.
After the introductory scene, Kyle Frost (Killian Coyle) prepares to unburden himself to Detectives Alison 'Tosh' McIntosh (Alison O’Donnell) and Ruth Calder (Ashley Jensen) about Lisa Friel (Tara Lee). He describes Lisa as lively but akin to a ticking time bomb, necessitating his severance from her due to her overwhelming presence. When Ruth and Tosh confront Lisa, who learns of their intention to search her van, Reverend Alan Calder (Steven Miller) steps outside and catches up with the unfolding drama.
The detectives grill Lisa about Kyle, specifically the shooting incident that transpired during her Manchester escapades with him. Lisa deflects, stating that Kyle shared little about his activities and that their relationship has ended. She adamantly denies any knowledge of the firearm. The conversation pivots to Lisa's altercation with Annie on the fatal day, where Lisa concedes that Annie caught her with pills, sparking their dispute. Tosh warns Lisa that both her and Kyle's narratives will undergo scrutiny.
Upon exiting, Ruth and Tosh discover that Lisa's van is devoid of weapons. Ruth concludes that they need a roster of individuals who had potential access to the gun. Elsewhere, Michael Stack (Andrew John Tait) reassures Ian Bett (Robert Jack) over the phone, suggesting Noah seek professional help for his recurring nightmares. Michael later informs Noreen (Lesley Hart) of Ian's concern, and Noreen agrees to visit.
Detective Constable Sandy Wilson (Steven Robertson) reveals the discovery of a video from Friday night's mission party, where Kyle served as the DJ. Sandy highlights that Kyle received numerous calls throughout the day. Ruth and Tosh concur that Lisa remains their primary focus.
At HEM, Ailsa Forbes (Liz Ewing) expresses surprise at Lisa's aggression towards Annie, noting Lisa's associations with unsavory characters and her history at the mission. Michael arrives, mentioning his upcoming meeting with Clara, and reveals his limited interactions with Lisa, primarily with Angus. Alan, meanwhile, learns of Lisa's impending departure and senses her reasoning for the argument with Annie was more profound. Lisa confronts Alan outside, seeking the campervan from Angus Wallace (Jimmy Yuill), but her true intentions are revealed when she slashes Alan's face and warns him to stay away. She storms off, abandoning her plan to borrow the van.
Alan trails behind Lisa, his voice imploring as he begs her to let him attempt to lend a helping hand. Lisa, however, insists that she knows what fate has in store. Meanwhile, Sandy relates to Ruth and Tosh that no one at the commission was forthcoming about discussing Lisa or Kyle. Their conversation drifts to how Lisa hadn't reached out to Kyle in nearly four months. Ruth receives a call from Alan, and upon visiting him, he reveals that Lisa urgently needs to speak with her.
Lisa clarifies that there was no argument at Hem; she was merely lashing out at Annie. She concedes that Annie had intended to assist her, but her frustration got the better of her. Annie had uncovered something that Lisa should have foreseen but hadn't. Lisa confesses that she's had Kyle's gun since he entrusted it to her just before they departed Manchester. Kyle had wanted her to dispose of it, but she saw it as a potential safeguard. She alleges that Annie took the gun that fateful day, fearing Lisa might do something rash with it.
Tosh expresses concern about Lisa's wellbeing, sensing that something untoward may have transpired, prompting her to confide in Annie. When Tosh inquires about the identity of this person, Ruth assures Tosh she has read everyone's file and is there if Lisa wishes to confide in her. Tosh declines, but Ruth suggests they confront this individual.
They confront Michael at HEM, while Ian recounts to Noreen how Noah behaves upon waking. Noreen proposes a day trip to cheer them up. Elsewhere, Tosh and Ruth converse with Harry Lamont (Tibu Fortes) about Lisa's allegations. Supposedly, Michael let himself into Lisa's room and assaulted her. Lisa had confided in Annie, who intended to report it to the police. They now lean towards believing the murder was an opportunity seized rather than premeditated. Harry underscores the need to prove Lisa's assault first.
Michael vehemently denies the accusation, asserting that Lisa's word cannot be trusted. He admits being at Hem until around seven that night but denies entering Lisa's room. He recalls meeting Frank Kirk for a drink; Frank was someone he knew from the council. Michael hints that Lisa might have falsely accused others too. He learns that Lisa had told Annie on the day she died, but Annie hadn't contacted him.
During a break, Tosh and Ruth deliberate on the case. Tosh wonders what motive Michael would have had to kill Anton Bergen. For that scenario to unfold, Michael would need to know that Annie had discovered the rape, which he couldn't have unless Annie disclosed her plans after Bergen called her. Tosh agrees to speak with Frank Kirk (Richard Thomson), while Ruth will verify Michael's alibi for the night of the murders.
Ian spends time with Noreen, discussing Annie and the myriad events unfolding. Meanwhile, Noah wanders unsupervised outside, eventually entering a house, the door clicking shut behind him. Tosh interviews Frank, who confirms he was with Michael that night, speculating they visited The Ship and Reynolds. Tosh suspects otherwise, believing Frank might have been at the mission.
Tara Kirk, alias Nalini Chetty, interjects their hushed discourse, eager to unravel the mystery unfolding. Frank concedes he might have briefly dipped into their mission but his visit was fleeting; he had left Michael behind in town. He justifies, citing the lateness of the hour as the reason Michael refrained from disturbing Noreen and Finn's slumber. As Tosh departs, she hastily dials Ruth to relay Frank's revelation that Michael had retreated to Hem after their pub visit, intending to spend the night there. Sgt. Billy McCabe, portrayed by Lewis Howden, informs Ruth that Michael and Noreen were questioned the morning subsequent to the grim discovery. Michael had claimed to be working late, attending to housing clients, with Sandy confirming his last appointment at 9:30 PM. He supposedly retreated to his council office thereafter, alleging to have toiled there till around 2 AM. Billy admits they hadn't verified his claims. Ruth urges Billy to contact the council offices to authenticate Michael's statement or delve into his whereabouts vis-à-vis Wethersta.
Unable to reach Michael, Noreen's anxiety spikes when she realizes Noah is missing. Ian ventures out in search of him, locating Noah's binoculars but not the boy himself. Entering a nearby structure, Ian discovers Noah in a state of panic, his words a jumbled confession of an incessant torment that won't abide. As they approach their vehicle, Ian informs Noreen of their impromptu change of plans.
Returning to the office, Tosh receives an update. Ruth reveals that Michael phoned Noreen at 10:54 PM on the fateful night. Sandy adds that Michael made a debit card transaction at the Black Road garage at 10:26 PM, placing him in proximity to Annie, given the party's vicinity, a mile away. Sandy promptly calls the garage to inquire about CCTV footage.
When grilled, Michael concedes returning to Hem, asserting he occasionally naps in the staffroom. He firmly denies encountering Lisa that night. Ruth probes his claim of working late on the night of Annie and Anton's demise. He maintains he was assisting a family in dire straits. When pressed if he had stepped out from the office, Tosh reveals his presence at the Black Road petrol station in Brae at 10:26 PM, prompting Tosh to suggest he might have tracked Annie and ended her life that night. Michael protests his innocence, blurting out that it wasn't rape before clamping his lips shut.
Despite Noreen's misgivings, Ian conveys Noah back to the croft. Inside, Ian reassures Noah of his safety, but Noah's fear overpowers him, causing him to flee outdoors. Noreen rushes to comfort him. Harry cautions Ruth and Tosh that they're far from solidifying Michael's presence at the croft during the murders. Billy's interruption brings news that Noreen informed him of Ian's decision to take Noah to the croft. Ruth's arrival is timed with the sound of thudding within. Ian claims he's merely aiding Noah in his terror. Ruth attempts to soothe him as Noah, eyes fixed on the house from afar, glances at Noreen, her hood pulled tight.
Tosh reviews the CCTV footage from the garage, calling Ruth to disclose that it was Noreen, not Michael, captured on camera. Ruth promptly orders a unit to the croft.
She urges Noah to fetch his father, proceed to her vehicle, and await her inside. Noreen announces her intent to depart, but Ruth intercepts her, sharing the gravity of Michael's situation. Ruth inquires if Noreen crossed paths with Annie that fateful day. Noreen reveals she popped by on her return journey from Hem, where Annie revealed Lisa's allegations against Michael. Realizing the urgency, Noreen knew she had to dissuade Annie from speaking out. Spotting Annie in conversation with Tosh, Noreen feared it was already too late. She trailed Annie and Anton to the safety deposit box, her memories flashing back to Annie placing Noah in a bathtub, with a gun in close proximity. Noreen insists her entry was solely for dialogue. As Annie dialed Euan Rossi, Noreen confronted her, who admitted to saying nothing to Tosh but hinted at possible future disclosure. Noreen endeavored to make Annie forget the entire ordeal and move forward. Suspecting Noreen's awareness of Lisa's truthfulness, Annie declares their intention to visit the police the following morning.
In a moment of desperation, Noreen seized the gun. Annie lunged for it, resulting in a fatal shot. When Anton entered, Noreen fired again, ensuring his silence with a second, decisive shot. Noah, hearing the gunshots, was left terrified. Noreen relates her subsequent flight, casting the gun into the ocean before returning home. Ruth confronts Noreen about her cruelty towards Noah, asking if she ever considers Lisa's suffering, to which Noreen responds negatively. Additional officers arrive to apprehend Noreen.
Later, Ruth informs Lisa of Noreen's arrest for the murders, revealing the submission of incriminating evidence and Lisa's testimony against Michael to the Fiscal. Lisa expresses concern for their unborn child, which Ruth gently reassures is not her burden. Alan and Lisa step outside, where Angus informs them of a spaceport inspection. Lisa expresses gratitude to Alan. As they depart, Alan learns that Ruth plans to give his dinosaurs to Noah and invites him for dinner. Meanwhile, Ian tells Ruth they are bound for Edinburgh, citing a letter from Rossi detailing Annie's past deeds. Ruth speaks to Noah outdoors, presenting him with the dinosaurs and discussing their upcoming journey. Noah shares that he had the dream again but this time, they escaped.
Elsewhere, Tosh encounters Tara and Joni but declines their invitation for a drink, preferring solitude with her order. Ruth joins her shortly, sharing the story of the dinosaurs.
