The episode kicks off at a bustling bus stop in York City, where the Bootham omnibus arrives, prompting a flurry of passengers, among them a man racked with cough. As his cough escalates into a violent fit, he reaches out to activate the emergency stop button, only to collapse onto the bus floor in a heap. The driver swiftly brings the vehicle to a halt, while a fellow passenger rushes to Sam's side, discovering him bathed in blood. The onlooker, startled, bolts off the bus on foot.
At the scene, a first responder hands DI Bea Metcalf (Laura Fraser) a protective mask. Dr. Loretta Parsons (Liza Sadovy) informs her that the deceased suffered from hemorrhagic shock, possibly tied to tuberculosis. Elliot Scott (Tom Lewis) draws Metcalf's attention to the victim's fingertips, notably devoid of fingerprints. DS Jake Hunter (Nathan Welsh) inquires about Patience Evans' (Ella Maisy Purvis) whereabouts, to which Metcalf replies they haven't spoken since Patience's birthday party, gently urging Hunter to mend fences with her. Subsequently, Metcalf presents Patience with a copy of "The Science Behind Interaction & Relationships," seeking perhaps to bridge their gap.
At the morgue, Parsons reveals to Metcalf and the team that the victim did not have TB. Patience interjects, asking if zoonotic infections were ruled out. Parsons assures her she is conducting the tests now and discovers blood in the mediastinum. Patience identifies this as a hallmark of inhalation anthrax, prompting Metcalf to demand expedited results. Parsons attempts to reassure her, clarifying that anthrax in the lungs isn't contagious through touch. Patience corrects her, emphasizing its transmission through infected animals.
DC Will Akbari (Ali Ariaie) warns Metcalf and Patience that the video found on the victim's phone is disturbing before pressing play. Disgusted, Patience abruptly exits the room. The footage, captured at a slaughterhouse, raises alarms. Hunter suggests a thorough examination of the facility. Shortly thereafter, the slaughterhouse manager (Liam Smith) demands an explanation for the APHA shutdown. Metcalf confronts him about the abusive treatment of cows depicted in the video, to which he responds with indifference. She places a photo of the victim on the table, and he identifies him as Wesley James Hutton, one of his employees.
Hunter later informs Metcalf that Hutton may have been using a stolen social security number, having lost his wallet a month prior. Meanwhile, the Climate Defense Force (CDF) posts a video titled "Would you treat your dog like this?" online, drawing attention to animal cruelty. Back at the morgue, Parsons informs Metcalf that anthrax tests at the slaughterhouse came back negative. Metcalf, feeling unwell, listens as Hunter briefs her on the CDF—a terrorist organization targeting companies perceived as environmentally irresponsible. They are believed to have orchestrated a fatal arson attack on a Dalton meat-processing plant five years ago. Hunter reveals that Superintendent Zara Blackwood (Flora Montgomery) has restricted access to the CDF file, noting that CDF founder Jean Crick (Cat Simmons) was charged with the arson.
Blackwood declines Metcalf's plea to access the confidential Dalton file, her tone dripping with sarcasm as she simply replies, "Because she said so." Soon after, DCI Calvin Baxter (portrayed by Mark Benton) issues a stern order to cease the investigation altogether. Meanwhile, Metcalf discovers Patience engrossed in the novel she had gifted her, and together they realize the Dalton case file is nowhere to be found. Metcalf contemplatively inquiries about the possibility of bending the rules, prompting Patience to inquire why she persists in seeking the file now that it's officially closed. Metcalf admits to being unable to shake the feeling of unfinished business, driven by an insatiable curiosity.
Later, Metcalf is jolted awake by an urgent call from Patience. Arriving at the criminal records department shortly thereafter, she finds Patience meticulously laying out the Dalton case on an investigation board, revealing that the sources used aren't from the missing file itself. The arson victim is identified as Dexter James, while Roland Mitchell, another worker, has been reported missing since the inferno, according to a filing by Annie Mitchell (played by Sonia Ritter). Metcalf speculatively wonders if counterterrorism had its eyes on the Climate Defense Force (CDF) around the time of the arson. Patience adds that a year later, 412 grams of a bioagent were seized from a lab during a raid, fueling Metcalf's suspicion that the CDF intended to use anthrax to contaminate the meat industry.
Metcalf ventures to visit Jean Crick in prison, seeking recognition of the Bootham omnibus victim. Through an electro-larynx device, Jean issues dire warnings of an impending environmental disaster and labels Metcalf a servant of a morally decaying system. Metcalf counters that Jean's involvement in the Dalton arson attack betrays any genuine concern for humanity, provoking Jean into a fit of distress. Metcalf swiftly alerts the prison staff. Soon, Hunter informs Metcalf that Roland Mitchell has been positively identified as the victim.
Visiting Vicky Crick (Sabina Arthur), Jean's sister, Metcalf learns that Roland and Jean share a son named Noah. As she exits the bathroom, Metcalf catches a glimpse of the man who fled the omnibus in one of the bedrooms. In a dramatic leap, he dashes out of a window, leading Metcalf on a relentless chase until he knocks her unconscious. Hunter and Will arrive promptly to apprehend the fugitive. Despite the ordeal, Metcalf insists on leaving the hospital that night.
Elsewhere, Blackwood grills Sam Fincher (Shane Redondo) about Roland's demise, to which he vehemently denies involvement. As the interrogation concludes, he reveals that Jean had asked him to keep tabs on Roland and ensure his safety. The following morning, Baxter confiscates Metcalf's warrant card, informing her of her suspension. She entrusts Hunter with the task of contacting Annie, while Baxter and Blackwood theorize that Jean orchestrated Roland's murder. Hunter disputes their hypothesis, but Blackwood insists he let it go.
Hunter pays a visit to Annie, who is adamant about seeing Roland. He unveils the crime scene photograph, prompting her to declare that the man depicted is certainly not Roland. Meanwhile, back at their home, Metcalf suddenly coughs up blood and collapses to the ground, leaving everyone in a state of shock.
Later, at a meeting for an autism support group, Patience receives an urgent call from Hunter, informing her of Metcalf's suspension. As the meeting progresses, Billy Thompson (played by Connor Curren) encourages Dev (portrayed by Darren J. Bransford) to speak up. Hunter then approaches Patience, relaying Annie's insistence that the omnibus victim is not Roland. He further reveals that APHA Trevor Willett has informed him of two more anthrax cases tied to the omnibus incident. Reluctantly, Patience agrees to review the CCTV footage of the omnibus.
Upon watching the footage, she notices that the victim was vaping, placing all patrons of the omnibus at risk. With urgency, she rushes to Metcalf's apartment and uses a hidden key to gain entry. Calling 999, she lies beside the unconscious Metcalf on the floor, waiting for help to arrive.
Metcalf is eventually rushed to the hospital for treatment, while Hunter remains determined to uncover the perpetrator. Patience inquiries about the possibility of obtaining the vape pen for further investigation. Elsewhere, Elliot apologizes to Patience for his earlier invitation, and she admits to having feelings for him but is unsure if she is physically attracted to him. She expresses her concern for Metcalf's wellbeing, and Elliot reveals that Parsons has access to restricted files under the Public Health Act.
Their conversation takes a tender turn as Patience kisses Elliot on the cheek, leading to a passionate kiss between them. At the morgue, Hunter asks Parsons to test the vape pen to see if it matches the anthrax strain found in the victim. Parsons mentions that out of the 412 grams of anthrax, only 350 grams can be accounted for, warning Hunter that conducting such a test could jeopardize her job. Hunter reassures her that her respirator mask has saved her from harm, and she signs out the Dalton file for further examination.
Patience later delves into the Dalton evidence, while Will discovers a CCTV camera outside the Crick flat. In a separate scene, Hunter visits Jean, who appears to be in the hospital. She struggles to communicate something important to him, adding an air of mystery to the unfolding events.
Hunter recounts to Baxter the access they gained to the Dalton file, prompting Baxter to宣布 their suspension immediately. Patience, sensing the gravity of the situation, implores Baxter to at least hear their evidence. Reluctantly, he concedes, granting them a mere two minutes of his time. Hunter unfolds a photograph of the authentic Roland, while Patience reveals that the omnibus victim is none other than Tyler Rose, an undercover police officer who had been actively investigating the CDF case. Baxter posits that Rose's cover was compromised, leading to his untimely demise.
Jean steps forward, claiming a romantic entanglement with Rose, who had agreed to abandon his police duties for a fresh start with her. Rose had once worked at the Dalton meat processing factory, where he and Jean orchestrated its destruction. To frame Roland, Rose left his titanium ring with him, ensuring his identity would be misconstrued. Post Jean's incarceration, Rose resumed his undercover role at CDF.
Later, Baxter confronts Blackwood about the recent installation of CCTV cameras on CDF members, a measure taken four months ago following an anonymous tip. Hunter lays surveillance photographs of Rose and Noah on the table, but Blackwood remains defiant, declining to comment. Meanwhile, Patience observes the interrogation through a two-way mirror when Metcalf approaches. Overwhelmed with emotion, she initiates an embrace, only to find Metcalf subdued and distant.
Hunter presents surveillance footage depicting Blackwood rummaging through a trashcan near the Crick flat. Baxter inquiries about her search, yet again she refuses to divulge any information. He then produces an envelope containing four unopened vape cartridges sent by Z Rose to Roland. Patience unearths a marriage license documenting Rose and Blackwood's union, suggesting that Blackwood's jealousy knew no bounds upon discovering Rose's affair with Noah.
Elsewhere, Alfie Metcalf-Haynes (played by Maxwell Whitelock) amuses himself with Patience's pet bearded dragon. Metcalf confides in Patience about his new medication regimen and his enrollment at a different school. He sparks her curiosity by inquiring about Mathilde Evans' (played by Lotte Lauren) enigmatic puzzle box. A flashback ensues, depicting toddler Patience (played by Oona Van Harneveldt) enraptured by Mathilde's piano melodies.
Returning to the present, Patience ponders whether the puzzle box holds the key to her mother's abandonment. With trembling hands, she opens it to find it empty, save for a mysterious number inscribed on the back of the label.
