Where do we start? It's always possible that someone could just bump into the guy who killed her brother - as you do, in a supermarket car park - and then that you get more and more obsessed with messing his life up. He's "done his time" but that's not enough for Cathy... She has to ruin the killer's life.
The story-line requires that working people just take time of of work; Cathy just disappears for an hour or two and her husband accepts it; a phone gets stolen, with no induction how. The dialogue doesn't flow, the characters are two-dimensional and uninteresting. The story gets more and more ridiculous and the demands on the poor cast become overwhelming: at times, you can read it in the actors' eyes: "How did I get involved in this?"
Fortunately, there are only four episodes to endure, although I suspect they planned for six and decided to cut and run after four.