Under salt marshes is set in Morfa Halen, a fictional Welsh town (which means “salt marsh”). Nestled between mountains and the unforgiving sea, the close-knit residents are threatened by their immediate surroundings.

Reilly’s character, Jackie Ellis, stars as a former detective turned teacher. As a storm approaches Morfa Halen, Ellis discovers the body of one of her students, an eight-year-old named Cefin.

The presumed drowned boy sets the town in turmoil with theories and digs up a three-year-old unsolved case that cost Ellis her career – and at the center is the disappearance of her niece.

Ellis’ estranged former partner Detective Eric Bull returns to Morfa to help solve the case. The pair are aware of two things: first, the cases are connected; second, they must solve the mystery before the storm arrives and destroys all evidence in its wave of destruction.

This is the debut series from writer, creator and showrunner Claire Oakley, almost five years after her success with the debut thriller Form.

Oakley says: “Under salt marshes is a crime story set in a community that is deeply connected to its environment and in which the worries, fears, dreams and secrets of its people are interwoven.

“At the heart of it all is Jackie, a character who embodies the wildness of the place we’re portraying. She’s free-spirited, compassionate and determined, with an iron resolve that makes her a true countrywoman. We’re absolutely thrilled to have Kelly Reilly bring Jackie to life.”

Under salt marshes will be broadcast on Sky and NOW. A release date has not yet been set.