As the curtains rise on the third installment of "Death Valley," Sioned Grant (portrayed by Amy Morgan) and Chris Grant (Steve Speirs) arrive at the wedding venue, their presence mingling with the impending sense of doom. A photographer (Rhys Taylor) and her assistant, Ashley (Aisha-May Hunte), begin capturing moments that will soon be shrouded in mystery. Within moments, Ashley rushes back, her face pale with the news: someone lies dead. Helen Baxter (Alexandria Riley) and Detective Sergeant Janie Mallowan (Gwyneth Keyworth) confer at the grim scene, discussing the victim, Gethin Bennion (Martin Gordon). Their suspicions whisper of this being a secret teenage drinking den.
DCI Barry Clarke (Steffan Rhodri) arrives tardily, bearing praise from the Chief Superintendent for their recent clearance rate, though Janie claims the accolade as her own. Clarke learns that the victim was the best man, who ran Alto Ignite Digital Marketing alongside the groom. Helen reveals that Gethin met his demise sometime between late last night and early this morning, while lodging in the rental cottage with the groom's entourage.
The groom, Aaron James (Kiell Smith-Bynoe), and his brother Nathan James (Benjamin Sarpong-Broni), stand amidst the turmoil. Helen recounts how Gethin’s head struck a gravestone with fatal force. DC Evan Chaudhry (Rithvik Andugula) canvasses the crowd for witnesses, while Janie inspects a shattered glass panel on the church's side door, believing the murder to be a meticulously staged affair.
In conversation with Nathan and Aaron, Janie learns that Aaron intends to reshuffle the wedding rather than cancel it, revealing this to be not just a wedding but a digital marketing super event, drawing clients from afar. Sioned, who invited potential clients to showcase her local gallery, is unperturbed by the digital twist. Janie probes further into Gethin’s character, with Aaron noting his odd behavior and seclusion in his room yesterday. Nathan, who spent the night confined to his own room, had no encounter with Gethin. Aaron mentions hearing Gethin around half-past midnight, when he was running water in his bathroom.
Meanwhile, John Chapel (Timothy Spall) expresses concern over Alan’s untouched breakfast, sensing something amiss. Upon learning of Gethin’s demise, Janie seeks his expertise via a message. Yvonne Mallowan (Melanie Walters) joins them at the pub, as Janie recounts the details of the bride, groom, and the fallen best man. John demands deeper insights into Gethin’s demeanor, finding it odd to proceed with the wedding amidst such fresh grief. He ponders the significance of Gethin’s speech, noting one card missing – a detail Janie initially attributes to carelessness but John dismisses, believing Gethin too meticulous to lose such an important piece. Janie recalls Nathan having a printed speech, raising questions about his preemptive preparation unless he foresaw himself stepping into Gethin’s shoes. Determined, John decides to accompany Janie and Yvonne, using the journey to confront Alan.
Just before their departure, Janie bumps into Rhiannon Hopkins (portrayed by Remy Beasley). Outside, John senses a deeper connection between them, sparking his curiosity. Yvonne, venting her frustration, laments Rhiannon's actions towards her daughter. They proceed to leave Yvonne behind, with John and Janie scooping up the cat before making their way to the church. Upon arrival, Hywel (played by John Weldon) immediately spots John and recognizes him from his television appearances. He gripes about the police disturbing the badgers, who have retreated underground ever since the murder. John dismissively remarks that they naturally live underground.
Janie confronts Nathan, who appears agitated. She inquires about the speech he delivered on the wedding morning, to which Nathan claims he penned weeks in advance, anticipating Aaron might change his mind. Janie remains skeptical. Nathan reveals he had hoped Aaron would ask him to be best man and confesses neither he nor Sioned cared for Gethin. Meanwhile, John visits the vet and then heads to a nearby gallery, where he engages Sioned Grant about one of the displayed pieces. Recognizing the artwork as hers, Sioned takes a call about wedding arrangements, mentioning adjustments to the rose count.
Post-call, Sioned confides in John that the more pressing issue is the best man's absence due to his untimely death. Despite her indifference towards Gethin, she becomes emotional. They agree to immerse themselves in art to distract from the sorrow. Elsewhere, Janie wrestles with cleaning up the mess Alan left in his vehicle. Evan phones, updating her on his investigation into Gethin's finances. It transpires that two weeks prior, Gethin received two thousand pounds from a classic car company for hiring the wedding car, yet Chris Grant asserts he paid Gethin four thousand for the same service.
Janie visits Chris, recognizing him as the Carmarthen Carpet King. He acknowledges paying Gethin for the car, hoping for a deal, unaware he was being ripped off. Chris describes Gethin as sleazy and recounts a major fallout with Aaron's brother on the stag night, suggesting Gethin had it coming. Janie asks about his whereabouts on the night of the murder, and Chris explains he was staying with Sioned until after the wedding, whom he could verify his whereabouts until around nine. He adds that, though Sioned is an adult, he senses he's becoming a nuisance. Chris departs shortly after.
Later, John unveils the painting he purchased to Janie, who reminds him of her advice to steer clear of suspects. Undeterred, John believes they are at the gallery instead of the pub because Janie is avoiding Rhiannon. They discuss Gethin's deception of Chris and Aaron's relentless pursuit of the wedding. John finds Aaron's behavior perplexing and intends to delve deeper with Sioned later, despite Janie's objection to their continued interactions. Ignoring Janie's wishes, John meets Sioned, sitting for her as she paints his portrait. Aaron walks in, discovering her task and comparing it to a past controversy, hinting at unresolved tensions.
Sioned elaborates, recounting how Gethin assisted her in purchasing a pre-owned car last year, yet she was remarkably indecisive, much to someone's frustration. Aaron jokes, suggesting she must have toured every automobile showroom in Wales. This remark stirs an upset in Sioned, prompting her to retreat upstairs. Meanwhile, Aaron engages John in conversation about his industry's relentless 24/7 nature. Interrupted by a call from the caterers, Aaron hastily departs.
Sioned returns downstairs, acknowledging Aaron's perpetual demeanor with a sigh. At the police station, Janie informs Evan that the victim had been expelled from the university, tasking him with uncovering the reasons behind his expulsion. Simultaneously, Janie delves into the rift between Nathan and Gethin. She encounters Helen, who reveals that the victim suffered severe brain damage due to a fractured skull, indicating a violent head trauma. Janie notes discrepancies in the groom and his brother's claim of hearing Gethin's taps at midnight; Helen contradicts them, pointing out that the victim's watch, smashed during the incident, stopped at 11:30 PM. Additionally, a high concentration of laxatives was found in his system.
Janie senses she knows the culprit's identity. She confronts Nathan, who admits to giving Gethin laxatives with the intention of getting him out of the way. Nathan confesses to discovering the body and staging a robbery by breaking the church door and wiping away evidence. Janie suspects Aaron might have heard Nathan's actions instead. Though suspicious, she doesn't believe Nathan is the murderer. She inquiries about a missing card from the best man's speech, which Nathan denies taking.
Later, Janie visits John to inform him that Evan has contacted the university about Gethin's expulsion, uncovering that Gethin had been stealing and selling exam papers. John, who has been sitting for hours due to Sioned, reveals that time flies when they talk. Janie warns him against becoming too close to someone who could potentially be the killer, despite Sioned's affair with Gethin. John insists she isn't the killer but faults Aaron for prioritizing his career over his partnership. Janie counters, acknowledging that perfect marriages are rare.
John shares a poignant past, revealing he was so engrossed in work that he didn't realize his wife was terminally ill until it was nearly too late. He implies Janie knows little about human relationships, given her extensive engagement with television characters. Janie encourages him to confide in Alan. In a subsequent visit with Sioned, Janie probes into her relationship with Gethin. Sioned admits it ended months ago but kept it quiet due to embarrassment. She reveals she opposed Gethin being the best man and even asked him to step down, but they eventually resumed their affair. Sioned insists Aaron was unaware of their affair, a claim Aaron contradicts, dismissing it as a mere indiscretion they planned to address post-wedding with the help of counselors. Janie once again mentions Gethin's expulsion from the university, leaving the situation layered with intrigue and unresolved tensions.
She firmly believes that Aaron's fingerprints were omnipresent on the incriminating item. Aaron, acknowledging his involvement, concedes that he committed the act, but Gethin bore the brunt of the blame out of a sense of obligation to Aaron. Gethin had crossed the line by exploiting his newfound salary increase while shirking his duties. Aaron, compelled to assert his authority, scrutinized the best man's speech, which was peppered with humorous remarks about navigating married life without a 'cheat sheet.' He reveals that he merely extracted a card from his prepared remarks.
Elsewhere, John engages Hywel in conversation about badgers and joins the local observer group, inquiring whether Hywel possesses night vision cameras to monitor their movements. Hywel confirms that he does indeed have such equipment. Janie discovers John's meddling in the investigation once again, prompting an apology from John. He shares with her details about the Cwmishel Badger Watchers and their night vision camera trained on the churchyard. Janie and John spend a nocturnal vigil analyzing the footage, culminating in the sighting of Sioned.
In an interview room, Janie confronts Sioned about her visit to the churchyard to meet Gethin around the time of his murder. Sioned explains that Gethin had texted her urgently, requesting a meeting. She stealthily exited through her bedroom window to confront him. Gethin confessed his love for her and proposed eloping together. Sioned rejected him, asserting it was a mistake. In retaliation, Gethin threatened to expose their affair to Aaron.
Later, Janie informs John that they lack sufficient evidence to arrest Sioned. Chris has retained a more adept legal representative. John doubts Sioned's capacity for murder to protect her secret, believing instead that she craves a partner who adores her and would fulfill her every desire. He senses something amiss in her narrative, urging the cancellation of the wedding. Sioned is visibly upset upon Janie's arrival at the nuptial venue. Aaron insists they cannot cancel again, fearing it would appear unprofessional. Janie warns that their secrets are about to be exposed, specifically Gethin's affair with Sioned, to which Aaron admits prior knowledge. Janie intends to arrest Chris for Gethin's murder.
John contends that Chris, driven by a desire to compensate for past neglect and alcoholism, exerted immense pressure to ensure the wedding's perfection. He believes Chris, seeking solace in drink, stumbled upon the churchyard, a local teenager hangout, where he overheard an altercation between Sioned and Gethin. A bottle of alcohol found at the crime scene will corroborate Chris's presence. Chris eventually confesses to pushing Gethin, resulting in his accidental death.
Later, Janie endeavors to console John over his neglect of Elin, suggesting perhaps Elin chose solitude of her own volition. She encourages him not to deprive her of that agency. As Rhiannon enters, Janie asserts her intention to confront issues head-on, like an adult. She inquiries after Rhiannon's well-being before resuming her seat. Janie reveals to John that she switched their drinks, ensuring Rhiannon will spend the night redecorating the bathroom. John expresses a desire to dispose of his portrait.
