The truth is finally out. The six-part Sky crime drama Under Salt Marsh ended this week, and viewers finally learned who killed young Cefin and Nessa. The finale, which aired on February 27, 2026, brought a raging storm, a flood, and a confession from the most unexpected person in the village. Jackie Ellis (Kelly Reilly) and DCI Eric Bull (Rafe Spall) came face to face with the killer in the marsh, and the discovery of James Rees during the chaos helped crack the case wide open. Here is exactly what happened in the final episode and why the ending left the town of Morfa Halen forever changed .
The Storm Traps Everyone as the Search for James Begins
The finale opens with panic. Dylan Rees (Harry Lawtey) arrives at Jackie’s door asking if his younger brother James is there. James is missing. Jackie immediately notices Dylan is wearing a white hazmat suit, the same suit from the children’s drawings. She confronts him about the hair found on Cefin’s body, which matched his DNA. Dylan explains he has been cleaning up hazardous waste to keep it from spreading into the floodwater. He admits he and another person have been recycling toxic material at an old amusement park called Sunnyside. They crush the waste and mix it into concrete, which was then used to build the village sea wall .
Dylan insists he is innocent. He says Jackie never trusted him and would not have believed him anyway. The argument stops when Dylan’s van stalls in the rising water. Jackie follows him on foot as the storm surge begins swallowing the village streets. Meanwhile, Eric Bull makes his way to Sunnyside alone. He finds leaking barrels and a pool of water. He pulls out a child’s blue wellington boot and a lens from a pair of glasses. Someone attacks him from behind. A fight breaks out, and the attacker, wearing a hazmat suit, locks Bull inside a small room. Water starts pouring in. Bull has no phone signal and no way out .
James Rees Makes a Terrifying Discovery
While Bull screams for help, James Rees is wandering through the abandoned site. He hears the yelling and follows the sound, thinking it is his brother Dylan. Bull talks to James through the door, telling him about a childhood friend named James Cook who died when Bull was 14. He says James Cook was a “superstar.” James Rees connects with the story and says Cefin was a superstar too. James manages to open the door, freeing Bull just before the room fills completely. Bull tells James to get to safety, but James insists Cefin was there before, trying to prove something. Bull carries the boy through the flooded streets and hands him over to rescuers .
The discovery of James at Sunnyside proves the children were drawn to that location. It confirms the link between Cefin’s death and the toxic waste operation. Bull now knows exactly where to look for answers.
The Confrontation: Jackie Finds Mac in the Marsh
Jackie and Dylan track down Mac Jones (Brian Gleeson), the man who worked with Dylan at the waste site. Mac is married to Nisha, who is the sister of Nessa’s mother. That makes Mac the uncle of the first victim. Jackie questions him at his home. Nisha mentions Mac came home one night “boiling hot” and had lost his glasses. The timeline matches the night Cefin died. Mac grabs a knife. Dylan tries to stop him but gets stabbed in the stomach. Mac runs out into the marsh .
Jackie follows him. Eric Bull, who has arrived at the scene, wants to go after her, but Dylan begs him to stay. Jackie finds Mac standing in the water. He has already cut one wrist with the knife. He is desperate and ready to die. Jackie demands the truth. Mac confesses to everything .
Why Mac Killed Cefin and Nessa
Mac explains both children stumbled onto the illegal waste site at Sunnyside. They were exposed to hazardous chemicals. Nessa was hurt badly, and Mac believed she would not survive. He said he had “no choice” but to silence her. He did not want to go to prison. Years later, Cefin found the same site. His boots burned through from the chemicals. He panicked and ran. Mac caught him. Cefin screamed for Jackie, believing she could fix anything. Mac could not let him talk. He killed Cefin to protect himself and the secret operation .
Mac claims he thought he was helping the town by building the sea wall. He saw himself as a protector. In his mind, killing the children was a twisted form of mercy because they were already injured by the waste. He tells Jackie, “Nessa would have died from her injuries.” Jackie is horrified. She jumps into the water and holds his head under, nearly drowning him. Eric Bull arrives but does not stop her. After a moment, Jackie pulls Mac out and hits him. She stops herself from killing him, choosing instead to let him face justice in a courtroom .
After the Storm: Justice and Moving On
The storm finally passes. The village is flooded. Osian Northam finds his sister Carys dead in the water, a tragic loss for the community. Ned Bevan remains missing after fleeing on a boat before the storm hit. His father Solomon tells Jackie that Ned is a coward. Solomon admits everyone in the village knew things and stayed silent. He calls Jackie the only one who actually tried to find the truth .
Mac is arrested. Eric Bull tells his partner Jess Deng to be honest in her report about his involvement with Jackie. He says he will include his own mistakes and accepts that he may face consequences. Bull decides to leave the police force. He talks about joining Gareth Morgan on a trip to France to photograph wildlife. Jackie finally tells Dylan about her pregnancy. Dylan admits he already knew for months and was waiting for her to open up. They listen to the baby’s heartbeat together .
In the final scene, Eric and Jackie sit together overlooking the damaged village. They forgive each other. Eric asks her to send him a postcard. The series ends with a quiet sense of closure. The killer is caught, the truth is out, and two damaged people finally find peace .
Who Was the Killer? The Answer Was Hidden in Plain Sight
The reveal that Mac killed Cefin and Nessa shocked many viewers. Actor Naomi Yang, who plays DC Jess Deng, told Radio Times she was surprised too. But she said it made sense looking back. “Sometimes you’re shocked because they’ve pulled it out of nowhere, but this makes sense, and it’s really impactful,” she explained . Mac was always in the background, connected to the family, and had access to the waste site. His motive was not hatred but a desperate attempt to protect his life and his image as a good community member.
Creator Claire Oakley told Variety she wanted the killer to be someone “normal.” She did not want a sexual motive or a psychopath. She wanted viewers to think about what pressure could make someone do. Oakley said Mac kills the children to protect his reputation. He convinced himself he was doing good by building the sea wall. That twisted logic made him dangerous.
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What Happens to the Village?
Morfa Halen survives the storm, but the community is broken. The illegal waste dumping is exposed. The sea wall, built with toxic materials, will have to be investigated. The deaths of Cefin, Nessa, and August Antonov are finally explained. James Rees is safe thanks to Eric Bull. Dylan will recover from his stab wound. Jackie can finally move forward with her baby and her life. Eric Bull leaves policing behind. The ending does not promise a happy future, but it offers hope. The truth set them free, even if it cost everything to get there .
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