Warning: Major spoilers ahead

As the episode begins, Jackie Ellis, who is passed out asleep on her desk at home, dreams of a person lying lifeless on the ground amid marsh reeds. Running towards the body, she suddenly wakes just as she reaches them, not revealing their face. Gathering herself, she heads to bed, leaving scattered papers and books from her job as a schoolteacher on the table.

The next morning, the town of Morfa Halen comes alive, with builders getting to work on the sea wall to protect the area from the increasingly high tide. Jackie walks past the construction site to head to work at the school. In the playground, one child, Cefin, shows off a magic trick he’s learned to her.

Cefin shares with her that he’s had a nightmare again where he’s chased into the water and drowns, but she reassures him that fears sometimes chase you in the night, but if you chase them during the day, he’ll be alright.

A dishevelled man in his 50s, Osian, approaches the playground, ranting about poisoned air and water. Jackie and fellow teacher Nisha Jones quickly round-up the kids to get them into the school building. However, Cefin runs up to him, telling him by name to go away. Osian grabs the arm of the boy and refuses to let him go until Jackie steps in. He only appears to relent when she speaks to him in Welsh.

Jackie assures Cefin not to worry before sending him inside, but Osian tells her that the kids need to be worried. Angry, Jackie tells Osian that he can’t go grabbing nine-year-olds, but she deals with him with sympathy. Osian settles, giving her a handmade leaflet saying ‘people are poison’, begging her to understand. Jackie tells him to go home to his sister, who she’ll call so she knows to expect him. But she warns him if he doesn’t go home, she’ll call social services.

Osian retreats but leaves with a stark warning - there’s ‘no way out’ - and requests she come to a meeting he’s arranging.

After work, Jackie heads to a riverbank, rolls a cigarette from a leather tobacco pouch but doesn’t light it. Instead, she just smells it before putting it away as she looks out on the water, deep in thought.

That evening, Jackie and Nisha head to The Ship Hotel for a local community meeting. The locals are informed that the money they’ve raised for the sea wall to protect Morfa Halen isn’t enough. They are told that they’re “out of time” - an upcoming storm could see the entire town underwater and are urged to consider leaving the area for their safety.

The local residents, however, refuse to budge, believing a bigger sea wall will protect the town and, just as they survived five years prior, Morfa Halen will continue to exist. The meeting is then derailed when farmer Solomon Bevan arrives, bringing a flock of his sheep into the pub. Solomon rallies the crowd, saying they’re not going anywhere.

Getting some fresh air outside, Jackie is approached by a local youngster, Dylan Rees, who gets close to her. Finding somewhere away from the crowd, the pair begin to kiss.

Later that night, while rolling another cigarette, Jackie is walking through a pathway among the marsh, when she spots something bright red in the ditch water. Approaching it for a closer look, she’s horrified to discover it’s a body – Cefin’s. She runs into the water to get him out, but it’s too late. Calling emergency services, she discovers that they are unable to get to her as the high tide has closed the causeway that connects the town to civilisation. She is asked to wait for assistance as they send an emergency first responder from Morfa Halen.

At the Hill house, Cefin’s mum, dad and brothers are watching TV, blissfully unaware of what’s happened and thinking he’s just out late. Jackie makes her way to their house, covered in mud and dirt having pulled him from the water.

Shell, Cefin’s mum, welcomes her into the house, with her husband Danny quickly shuffling the kids off to get ready for bed as he can tell something’s wrong. Offering her support thinking something’s happened to her, they offer to call someone for her, but she breaks the news she found Cefin in a drainage ditch. Shell’s kindness quickly turns to rage and denial, thinking Jackie is lying or playing a prank, while Danny’s instinct is to head down there, dragging Shell out the door and leaving Jackie to look after their other two kids while they assess what’s going on.

Jackie, still shell-shocked from what she’s witnessed, is left alone in the living room, unsure of what to do next.

At the other end of the causeway, DS Bull waits, unable to get across due to the rising water. As his co-worker, DC Jess Deng arrives, she questions if the body is that of Nessa Taman – but he says no, as the victim is male.

Arriving at the scene, the pair find Cefin’s devastated parents crying over his dead body. He helps them out of the ditch, before speaking to Dr Taman – the first responder on the scene. She informs him that Jackie Ellis discovered the body, and he recognises the name. Asking where she is, Dr Taman isn’t sure.

As the police take the Hills back home, Jackie slips out, leaving them to grieve by themselves, and heads into the woods.

As the forensics team begin their investigation, Jackie collapses by a tree and begins to cry, having visions of a young girl playing on a swing in the reeds.

As Jackie walks along the waterfront, she finds Bull asleep in his car, panicking when she sees him. Deciding not to wake him up, she slips into her house and calls the police station, asking to speak to the Superintendent's office, adamant that Bull should not be handling the case.

Unfortunately for her, he’s woken up, and is now ringing on her front door. She refuses to answer it, and he leaves, heading back to the crime scene.

With just hours before a heavy rain forecast, Bull and Deng are keen to get as much evidence as possible before it’s washed away. Currently, there’s nothing to suggest it’s anything other than a tragic accident – except one handprint in the mud which raises questions, and the fact the blue wellies Cefin was wearing are yet to be found. Deng asks if Jackie has given a statement yet, and offers to do it in Bull’s place, but he declines the offer, saying he wants to do it.

At her home, Jackie uses a TENS machine to monitor her stomach, revealing a heartbeat – she’s pregnant.

When she heads to school, Nisha shares that she's decided not to tell the children about Cefin, instead sending letters home for the parents that evening. She comments that the death is “bringing everything back”. Looking through children’s drawings, she notes one made by Cefin of a person in a white body suit – like an astronaut? Later that day, they take the children to swim in the ocean, where Bull finds them.

Bull approaches James, Cefin’s friend, who is aware something is wrong. Bull asks why he’s not out with the rest of his class, and James shares he was stung by a bee, which has left a welt on his hand. Bull starts asking questions about Cefin, and James shares that sometimes they go and play on Solomon Bevan’s farm – Cefin’s granddad’s. Bull asks if James was with Cefin yesterday, but before he can answer a furious Jackie spots them and tells Bull to get away from him.

Jackie and Bull clearly know each other, and as she sends James away, Bull explains he needs to start taking statements from everyone, especially her as she was the first person to find the body. While Jackie tries to shrug him off, Bull tells her everything is police business, particularly when a kid ends up dead. Their conversation is overheard by James, who deduces they’re talking about Cefin.

Jackie bitterly tells Bull he’s not the right person for this investigation and tells him to stay away from the beach and from her. He tells her he’ll be back to get her statement at 3.30pm after school, and on the way back to the road runs into Dylan, who also seems unimpressed by the detective’s presence in Morfa Halen.

Bull continues his investigation by heading to the home of Gareth Morgan, head of the Wales Environment Institute and the man who had given the speech at the pub yesterday evening, urging people to leave the area. He recognises Bull, who explains he was previously on the Nessa Taman disappearance case three years prior, but Gareth notes he’s only lived in the area for the last six months.

Gareth openly answers Bull’s questions but can’t shake the feeling that they’ve met before. Bull dismisses him, asking his whereabouts and asking for an alibi. Bull notes that they found sea water in the ditch, but Gareth says that’s not possible as the ditches are meant to channel rainwater away from the town. He warns Bull to park his car uphill due to the rain due that evening, which is set to cause flooding.

Gareth then clicks that he met Bull three to four years ago, but it’s not specified where or how. Before Gareth can explain, Bull shrugs him off again and leaves, heading to meet Jackie at the school – but she’s already gone. He heads to her house but she’s not there. Instead, she’s at her dad’s house.

He’s sympathetic and concerned about her, but she assures him they’re going to be alright and asks if he’s told Lydia. He says that he didn’t want to concern her and asks her not to go back to the time of Nessa, because she’d been “doing so well”.

With no sign of Jackie, Bull heads to the Hill’s house, where he finds their entire living room filled with their family members. He explains he needs to pin down Cefin’s movements from 3.30pm (when school ended) to 7.30pm (when he was found) and will need to speak to everyone individually.

Solomon, who is sitting at the kitchen table, asks if an investigation means that Cefin’s death wasn’t an accident, but Bull assures him it was normal procedure. Solomon says that Bull destroyed families before with false accusations and tells him not to dare point a finger to anyone in the room. Reading the enmity on the room, Bull leaves his card for anyone to contact him and makes a swift exit.

That evening, Jackie heads to the fish n chip shop to get dinner, spotting Dylan outside with friends. The pair give each other a small nod but otherwise don’t interact. As Jackie speaks to Jim the Chippy, Bull arrives and demands a statement from her. She refuses to talk to him, but he says if she doesn’t speak to him now, he’ll take her down the station.

As Jackie leaves, Bull quickly follows, saying a statement in his car will do, but he’s stopped by a drunken Dylan, who confronts Bull for telling his nephew, James, that his best friend was dead. Bull tells him to walk it off and Dylan punches him in the face. Jackie steps in and says she was the one who James found out from, though it was an accident. Angered, Dylan walks off, and Jackie begrudgingly agrees to giving her statement in Bull’s car.

With the rain now fully kicking in, Jackie tells Bull everything she knows and explains why she decided to tell the Hills about Cefin’s death, rather than let the police do it. She also asks if they think it’s a murder or not.

Bull tells her she’s not police anymore and shouldn’t have involved herself in police business. Jackie snaps back at him, saying that sometimes things are personal. She claims once the Hills returned to their house, she went home – which Bull knows is a lie.

When he starts asking about Osian and the incident at the playground the morning of Cefin’s death, Jackie defends him as harmless. With the statement coming to an end, Bull pauses the tape and asks why he lied to Dylan for him – she responds by saying she wants justice for both Cefin and Nessa, and it would be easier to achieve that if he’s not hated in the community and able to do his job.

He responds by saying she never should have left the force, but she tells him it became impossible when her partner on the job – Bull – messed things up and betrayed her.

That evening, Bull joins Gareth and Danny Hill in the woods, despite the downpour. Gareth warns that they’re expecting a storm in the upcoming days that’s so big it could trigger an evacuation.

When he gets back to The Ship Hotel, owner Carys Northam makes him strip off in the middle of the bar instead of dragging in water. When he gets to his room, he gets a call from forensics with an update on the investigation – there was a hair Cefin’s neck that wasn’t his, and he had a bee sting on his neck. His eyes were also yellow, not white, and the water in the young boy's lungs didn’t match that of the drainage ditch – meaning someone moved the body.

As the town settles in for the night, Bull heads back to the crime scene and follows a drainage pipe that’s spilling water into the ditch. Jackie, meanwhile, retrieves a purple box filled with memories of young Nessa – her niece – and discovers she’d drawn a picture of a man in a white astronaut-like bodysuit, just like the one Cefin had drawn before he died.