Under Salt Marsh is set in the fictional Welsh town of Morfa Halen, a tight-knit community precariously nestled between towering mountains and a fast-encroaching sea that threatens its very existence. As a once-in-a-generation storm begins to gather far out at sea, former detective turned teacher Jackie Ellis discovers the body of her 8-year-old pupil, Cefin, seemingly drowned. The discovery sends shockwaves through the community, reviving the ghost of an unsolved cold case that rocked the town three years prior – the disappearance of Jackie's niece, Nessa, which cost her career.

Cefin's death summons Jackie's former partner, Detective Eric Bull, back to Morfa Halen to lead the investigation into a community he failed once before. Convinced the cases are linked, Jackie and Bull must reconcile and race to uncover long-buried secrets inside Morfa, before the storm breaks and all the evidence is gone for good. An intimate and cinematic story, Under Salt Marsh takes audiences into the heart of a community bound by nature and fractured by an unthinkable crime.