Warning: Major spoilers ahead

Picking up exactly where we left them in episode five, Jackie is sat stunned as Dylan comes through the door in a white hazmat suit – the same kind both Nessa and Cefin drew in the days before they were killed.

Dylan, however, is panicking for a whole different reason – he asks if his nephew James is there with her, having gotten wind that he’s gone missing.

Jackie pushes and questions him on the hazmat suit, but he avoids the topic, opting to focus on the missing James, but she’s not having it. He tells her he’s innocent, even lining himself with her sister, Lydia, who had also previously been falsely accused. Dylan insists he was just ‘clearing things up to keep it away from floodwater’, but that still doesn’t explain why a hazmat suit was required.

When she asks why he didn’t tell her that his hair was found on Cefin’s body, he says it was because she doesn’t trust him and never has. But the evidence is mounting up against him, and even he can see that. Cefin died in toxic water, after all, but it still doesn’t stop him from being offended that she could think he’s a killer. He storms off to find James, but Jackie chases after him in her car.

Up in the forest, a search party is sent out to try and find young James, but with rain pouring fast and branches falling everywhere, it’s becoming increasingly dangerous to continue. Cara is heartbroken, insistent that he couldn’t have gone far on his own.

Bull calls Deng to update her, but the signal is majorly disrupted because of the storm. He makes his way to Sunnyside Amusement Park – where the construction team for the sea wall were based – to try and find some answers. Bull makes his way through the abandoned buildings – but with lights on and loud noises, he knows someone else is in there.

Making his way into the basement of one of the old buildings, Bull discovers it’s a storage unit for barrels of toxic chemicals, some leaking, as well as unused parts of sea wall. Determined to keep moving, but knowing the danger, he grabs a large spirit level and continues his search until he finds a room with a hazmat suit hanging up. Next to it is a bathtub filled with water – and remnants of blue rubber, the same colour as the missing wellingtons Cefin was wearing. Spotting a glasses lens in the tub, he picks it up and takes it for evidence before moving on.

As the town continues to flood, Dylan is driving tire deep through the water on the hunt for James, with Jackie following closely behind. Eventually, Dylan is stopped in his tracks as a rogue wave crashes through his van window, literally flooding the engine so it can’t move. Dylan gets out and joins Jackie, telling her they need to leave and get uphill, but she refuses to move until he explains the hazmat suit.

With Dylan’s concern about James escalating, he finally folds and admits that they recycle toxic waste by mixing it into the concrete for the sea wall. He promises to tell her everything she wants to know as long as they get moving. She realises that’s why he told Cefin and James about the Black Sow myth – to scare them off the site – but Dylan is horrified when he clicks it means Cefin went to Sunnyside the night he died, as he had planned to confront the Black Sow to stop being scared of it. He swears he had nothing to do with Cefin’s death.

When asked about the DNA hair evidence, Dylan tells Jackie he never told her because she knows she doesn’t trust him. He tells her he knows she’s been pregnant and has been for months. He has been desperate for her to tell him, but she never did, and he takes it as a sign she doesn’t actually care about him. Jackie admits time with him is one of the few good things she has, and she didn’t want to ruin it.

Deciding to trust her gut – and trust Dylan – she asks who else could’ve worn a hazmat suit like that with his hair on it. He tells her his boss, Mac Jones, is the only other person who even knows about the waste.

As this unfolds, Bull is still lurking around Sunnyside for answers when he’s attacked by a man in a hazmat suit. Ripping off his mask in the fight, he discovers it to be Mac. Getting the upper hand in the struggle, Mac rushes out of the room and locks him behind a steel door – in a room rapidly filling up with water.

Dylan and Jackie head to Mac’s house outside the town and find Mac’s wife Nisha nursing their baby son. She tells them Mac thought it was safer there than the master assembly point in the forest or the Bevan’s farm. When Jackie asks about whether Mac was acting strangely the night of Cefin’s death, Nisha calls her crazy for making her husband her latest investigation target, and tells her to get out – but is left shaken when Dylan shares what they’ve been doing with the toxic waste in the sea wall.

Still thinking the better of her husband, Nisha insists Mac would never do what they’re claiming and demands they leave. But when Jackie begs her to think about the day Cefin went missing, Nisha begins to hesitate, and looking at her son gives her pause for thought. She does know something.

As the search party continues to look for James in the forest, things are looking dire for the town – and particularly Bull, who knows that he’s not got long before he drowns. He frantically starts banging on the pipes in the room, hoping someone will hear him.

Thankfully James, armed only with a wind-up torch, has made his way to Sunnyside to see Dylan, and despite being terrified, follows the sound of Bull’s voice to get him out. As he needs to keep talking, Bull shares a story about James Cook, his best friend when he was a kid. Bull admits he loved James, but he died at the age of 14 when he got himself into a dangerous situation and Bull was too scared to help him. He still thinks about him every day.

James gets to the door and opens it, with Bull – and all the water he was trapped in – falling out. Bull thanks the youngster but says it's dangerous for them to be here. James explains he had to, because Cefin was there, and he needed to prove it so he could help. Bull reassures him he did, and together they make their escape.

Mac returns home to find Jackie and Dylan waiting for him. Clearly already in a state of panic after his run-in with Bull, he offers them a drink. Jackie shares she knows about the waste in the sea wall, and he scolds Dylan for spilling their secret. Jackie then asks if he was involved with the landfill deal – but Nisha gets her own question in first.

The night Cefin died, he had arrived at the community meeting boiling hot and missing his glasses. She asks how he lost them. He tries brushing her off as he makes himself a stiff drink, banging at ice with a knife to chip some off. When Jackie asks him to put the knife down, he refuses, saying she’s treating him “life a f**king maniac”.

Scared for Jackie and their unborn baby, Dylan jumps in to get the knife off Mac, and is stabbed in the stomach during the tussle. Mac then makes a run for it, with Nisha going after her husband, while Jackie tends to an injured Dylan, who slumps on the floor in the kitchen.

Bull gets James to a rescue boat helmed by Danny Hill, and thanks the kid for saving his life. He then tells Danny to let Deng know where he’s going, takes one of the team’s boats and heads off to track down Mac.

Arriving at the salt marsh, he runs into Nisha, who has lost Mac amongst the reeds. When he takes her back to the house, he sees a severely injured Dylan being tended to by Jackie. Knowing Mac is somewhere in the salt marsh, Jackie heads out on her own, knowing the area better than the others. Dylan urges Bull to go after her and he does, leaving him with Nisha, who tends to his wounds.

With Mac, Bull and Jackie now all navigating their way through the marsh reeds, Jackie has a vision of Nessa, who guides her to a piece of open marshland. There, she finds Mac scrambling on his knees trying to find something in a pool of ditchwater. She tells him it’s over, but he refuses, picking up the knife he was looking for and slashing one of his wrists. He declares he knows she wants to see him dead, but Jackie denies it, saying she wants him alive and rotting in a cell instead, paying for what he did to Nessa and Cefin.

Mac begins to taunt her, revealing Nessa had screamed for her when she died – as did Cefin. He says he found Nessa after she’d hurt herself on the quarry, which at the time was still a secret from everyone. Cefin had found the waste on the building site, and it had burned through his wellies. When Mac tried to clean him up, he was crying and screaming, so he drowned him to shut him up as he had threatened to tell Jackie what was going on so she could help. He killed them both to hide his secrets, claiming he had no choice.

Jackie angrily says that they were family and he’d ruined everything, but he continues to push, insisting Nessa would’ve died from her injuries anyway. She hits back that he never gave her the chance, and lunges at him to get the knife off him. In the scuffle, Jackie holds Mac’s head underwater. Bull watches on – allowing Jackie to do what she will. Eventually, Jackie lets him up, hitting him across his face and laying him flat on the marsh.

As the rain finally stops and daybreak comes, the residents of Morfa Halen – who have taken shelter at the Bevans’ Farm – head outside to overlook their beloved village, which is now destroyed and mostly under water.

While inspecting the damage, Osian wades through the water to The Ship Hotel, and is devastated to find his sister Carys dead, having drowned after refusing to leave.

Mac is kept locked in Solomon’s Farm office, with Deng keeping a watchful eye on him. Bull tells her to be honest in her report of what happened, and to include his insistence to involve Jackie in the investigation – even if it means he would be stood down or even fired for gross misconduct.

Deng says she knows now it was for Jackie’s own sake that he allowed her into the investigation, and she believes that to be OK. Bull tells her to keep an eye on Mac until they can ship him to the mainland for psychological assessment. He’s admitted to murdering Cefin and Nessa, but could well also be responsible for August’s death too.

Elsewhere, Dr Taman is looking after a wounded, but alive, Dylan. Jackie asks to be checked too as she’s pregnant and wants to make sure the baby’s ok. Using a stethoscope, the doctor shares the baby’s heartbeat with Jackie, who then gives it to Dylan so he can hear as well. Dylan instantly melts, knowing they’re both finally at a place where she’s willing to be more open with him.

At the farm, Bull approaches Gareth, asking him about how bad the damage is not just in Morfa, but also other coastal towns. With the community having to wait for resources, Gareth asks if Bull’s sticking around. He says he will. Gareth shares that he doesn’t fly but hopes to go to France once this is all over to see all the birds that live in certain areas there.

As the relief efforts continue at the farm, Solomon talks to Jackie, and shares that Ned is still missing. He brands his son a coward for running away from what he did, but notes everyone – except Jackie – knew something was wrong but did nothing, offering her unspoken praise for her continued fight.

When Jackie spots Bull sitting alone, she goes to join him, and the pair talk about their hopes for the future. Bull wants to see more of the world (maybe starting with France), while Jackie is happy that her baby is OK and she’s in a better place with Dylan. Jackie requests wherever Bull ends up, that he send her a postcard.

With the pair finally getting the closure they’ve been craving for years over Nessa’s disappearance, the former partners separate with a nod. When Bull goes to join the community and get some food, he turns around to find Jackie already gone, leaving behind the peace and calm of the Welsh mountains.