The sixth episode of "Under Salt Marsh" begins with Jackie's shock and confusion as she confronts Dylan, dressed in a pristine white hazmat suit. Her immediate suspicion is raised by the presence of hair on Cefin's body, leading her to question whether Dylan could be the culprit. Dylan explains that he has been "cleaning stuff up," which partially justifies his attire. Jackie brings up the toxic waste and Nessa's drawing of a man in a hazmat suit, but Dylan becomes agitated and curses at her for her accusations. He decides to leave, more preoccupied with finding James, who is still missing.
As the townsfolk search the woods for the missing boy, Dylan drives into the heart of the town where the water is rapidly rising. In fact, a wave comes and knocks his truck right into a telephone pole. Meanwhile, Bull heads to the abandoned theme park, Sunnyside, to try and get some answers. When he arrives, he discovers barrels full of toxic waste and sea walls that have been constructed using this waste. It becomes clear that Sunnyside is being used as a spot where Dylan was recycling hazardous waste. They were crushing it and then putting it into the concrete sea walls.
This explains why Dylan mentioned the Black Sow story to Cefin and James. He didn't want the boys going near Sunnyside because of the waste. However, this had an adverse effect on Cefin, who went hunting and found the waste. A loud noise from deeper in the facility draws Bull into the darkness, and he not only finds hazmat suits but also the killer - Mac.
Dylan and Jackie eventually spoke plainly, where Dylan explained that the only other person with access to Sunnyside was Mac, who also wore a hazmat suit – sometimes the same one as Dylan. This explained why Dylan’s hair was found on Cefin.
Bull goes hunting but unfortunately, Mac gets the jump on him first and locks the detective inside one of the rooms. The sea level continues to rise, and things don't look good. Bull pounds on the vents and calls for help, and it's James who heeds the call. Turns out he’s been hiding out in Sunnyside, and he manages to help Bull out.
Meanwhile, Mac heads back home, where Dylan and Jackie both confront him about the sea wall. Dylan tries to protect Jackie when she boldly challenges Mac (who’s currently holding a knife) and ends up getting stabbed for his troubles. As Jackie tends to him, Mac rushes out of the building.
When Bull heads back into town, handing James back to Cefin’s dad, Jackie races into the marshes to find Mac alongside Bull. Jess is nowhere to be found at this point.
Mac slits his wrist when both Jackie and Bull catch up to him in the marshes. He explains that when he found Nessa originally, she screamed for Jackie. Cefin did the same thing when Mac killed him too. Both thought Jackie could fix the toxic waste issue.
Cefin clearly stood in the toxic waste when he arrived at Sunnyside because it burned through his wellies. He was in a panic and struggling, but Mac tried to help clean it up. However, as Cefin kept shouting and raving, Mac claimed he had “no choice” and killed the boy there and then. He also chalks Nessa’s death up to a “kindness”, believing she would have died from her injuries otherwise.
Jackie is enraged hearing this confession and grabs the guy, looking set to drown him in the marshes. Bull watches on, silently, until Jackie stops at the last second and decides proper justice should be served in a court of law.
After the storm's fury subsides in the morning, the townsfolk emerge from their makeshift shelter in the barn, their eyes widening in shock at the extent of the flood that has transformed their once-dry village into a sea of water. Despite warnings, the damage is overwhelming, a stark reminder of nature's power.
It takes a mere 38 minutes for Jess to arrive amidst the aftermath, while Bull keeps Mac sequestered away from the crowd. Bull confides in Jess that they must be honest in their report about his involvement with Jackie, knowing full well that his actions could lead to dismissal for gross negligence. However, he's willing to take that risk.
Jess understands Bull's decision to include Jackie, believing it's for her own closure and agrees with it. Mac's guilt over August's death remains unspoken but assumed.
As for the story's conclusion in "Under Salt Marsh," Jackie embraces her pregnancy and begins to open up about her relationship with Dylan, taking small but significant steps towards a new beginning. Meanwhile, Bull speaks with Morgan about their future plans.
Solomon opens the barn doors to welcome the villagers, recognizing the importance of community support during this difficult time. He notes with pride how Jackie is trying to mend things and tells her so. Sitting together, Jackie and Bull reconcile their differences, with the latter announcing his departure from the police force, whether temporary or permanent remains uncertain. He leaves her with a kiss on the cheek and a promise to send a postcard from wherever his journey leads him. The episode ends with a sense of hope and renewal, even amidst the wreckage caused by the storm.
