The killer in the British crime thriller Under Salt Marsh has finally been unmasked, and even one of the show’s own stars admitted she didn’t see it coming. Naomi Yang, who plays Detective Constable Jess Deng in the Sky Atlantic series, said the identity of the person who murdered nine-year-old Cefin and, years earlier, young Nessa left her shocked. But in a recent interview, she explained why the reveal, while unexpected, made perfect sense when looking back at the clues.
The six-part drama, set in the fictional Welsh coastal town of Morfa Halen, has kept audiences guessing for weeks. The story follows former detective turned teacher Jackie Ellis, played by Kelly Reilly, who finds the body of her pupil Cefin in a drainage ditch. The case forces her to reconnect with her old police partner, Detective Eric Bull (Rafe Spall), and his new partner, DC Jess Deng (Yang). As a massive storm cuts the town off from the mainland, the investigation uncovers toxic secrets, illegal waste dumping, and a connection to the unsolved disappearance of Jackie’s niece, Nessa, three years prior.
The Killer Was Hiding in Plain Sight
WARNING: Major spoilers for the Under Salt Marsh finale ahead.
In the gripping final episode, the truth comes out. The killer is Mac Jones, played by Brian Gleeson. Mac is married to Nisha, who is the sister of Nessa’s mother, making him Nessa’s uncle by marriage. He was also involved in a scheme to recycle toxic waste at the Sunnyside amusements site, crushing the hazardous materials and using them to construct the town’s new sea wall .
Both children met their fate after accidentally stumbling upon this illegal operation. Years apart, Nessa and then Cefin were injured by the toxic waste. Mac found them on both occasions. Believing they had seen too much and were already fatally injured, he made the decision to kill them to protect the secret and, in his twisted reasoning, his family and the town .
Mac confessed to Jackie in the marsh as the storm raged, admitting he felt he had “no choice.” He was eventually apprehended by police, bringing an end to the mystery that had haunted Morfa Halen .
Naomi Yang: “It Makes Sense”
For Naomi Yang, whose character Jess Deng worked tirelessly to solve the case alongside Bull, the reveal was a surprise, but not a cheat. In an interview with Radio Times, she explained why the writing worked so well.
“I think it’s always a sign of really good writing when you’re shocked, but then immediately you’re like, ah, it makes sense,” Yang said. “Sometimes you’re shocked because they’ve pulled it out of nowhere, but this makes sense, and it’s really impactful” .
Yang admitted that the show did a great job of spreading suspicion among the town’s residents. Throughout the series, viewers were led to suspect several characters, including the local oddity Osian, the reclusive beekeeper Kieran, and even Dylan, the boyfriend of Jackie and uncle to Cefin, after his hair was found on the boy’s body .
“You knew there was someone in the community, and throughout the series you’re like maybe it’s Dylan, maybe it’s this person, so it caught me by surprise, but also I was like ah, I see it,” she explained. She contrasted the reveal with other shows, adding, “Gossip Girl, there was no clue there, but with Mac, it makes sense” .
Understanding Mac’s Motives
While Mac’s actions are undeniably unforgivable, Yang also pointed out that the character believed he was acting for a greater good, which adds a layer of tragic complexity to the villain.
“The lengths people are willing to go to, he’s trying to protect the town,” Yang expanded. “It’s coming from this place of–he could have obviously gone about it a different way, but it’s coming from a place of love for his home and wanting to raise his family there” .
She described his final confrontation with Jackie, noting his desperation. “You can see he’s desperate, he doesn’t want to face what he’s done because there’s too much. But that wasn’t his intention. He’s trying to, in his own weird way, do a good thing for him and his family and the town. But he got lost somewhere on the way” .
Jess Deng’s Journey and Growth
The investigation also served as a major turning point for Yang’s character, Jess Deng. Starting as a recently promoted detective and an outsider in the tight-knit community, she had to navigate the emotional baggage that her partner, Bull, brought to the case .
Yang noted that the dynamic between Deng and Bull evolved significantly. Initially, Deng was prepared to follow orders from her superior. However, as Bull’s personal history with Jackie and the town came to light, Deng had to step up .
The finale features a crucial scene between the two detectives. Bull admits that Deng was right to involve Jackie and to push for the truth, even if it meant bending the rules. In return, Deng acknowledges that she understands why the case was so personal for him.
“And I think by the end, it’s so beautiful when they have that last scene, because he’s sort of meeting her where she is, of like, okay, you do your report [ignoring protocol and involving Jackie], you were right. The truth needs to come out,” Yang shared. “And for her, she’s meeting him halfway. She’s like, I get it. It’s personal” .
Yang admitted she “never expected” their relationship to reach that point of mutual respect. “They’re both quite strong willed, which is why they don’t really work at the beginning. But that moment where they meet in the middle, that’s huge growth for both of them,” she said. “I might not agree with it, I might not think it’s right, but I can meet you halfway” .
The Atmosphere of Morfa Halen
The show’s success is also due to its atmospheric setting. Filmed in North Wales, including locations like Barmouth, Fairbourne, and the stunning Newborough Forest, the environment became a character in itself .
Yang previously spoke about how the landscape shaped her performance. “Being by the sea, surrounded by the hills and mountains of the Eryri National Park, you can’t help but be deeply affected by your surroundings,” she told 1883 Magazine. “I felt almost engulfed by the energy and vastness of it all. That really helped inform how I wanted Jess to feel – that she was coming into this world as an outsider and the place feeling at the same time both impenetrable and overwhelming” .
The brutal filming conditions, complete with rain machines and freezing temperatures, added to the authenticity. Yang recalled being “soaked through” and standing next to heaters with co-star Mark Stanley, “literally steaming” .
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What Happens After the Storm?
The finale of Under Salt Marsh ties up the central mystery but leaves the characters forever changed. With Mac arrested, Jackie can finally begin to heal. She reveals her pregnancy to Dylan, who admits he already knew and was waiting for her to be ready to tell him .
Bull informs Deng that he will include his own mistakes in his report, signaling his intention to leave the police force and possibly join his love interest, Gareth, on a trip to France .
For the community of Morfa Halen, the storm has passed, but the town is flooded. The residents must now rebuild, not just their homes, but their trust in one another, after discovering that one of their own was capable of such terrible acts .
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