Kath is emerging from her grief about losing her sister and goes on an emotional journey to try to find out what happened to her.
Shirley Henderson is wonderful in the role of Kath. The supporting cast is superb. It's beautifully shot. The ending is ambiguous, I have worked out what I think happened but others might disagree. It is not simply a "whodunnit" or even a "whether anyone dunnit" but a study of a young woman in pain and her pain is eventually transferred to someone else. That makes it sound a bit gloomy and I wouldn't claim it's an easy watch but it is a fascinating one. I would watch anything by this director again.
I understand why Fleetwood was chosen as the location but it is not as threatening as it might appear. Dangerous if you don't know what you're doing but actually a fascinating and quite a pretty place. You can also visit the trawler used in the filming, a real, old, working ship, now come home to Fleetwood.