It's a dark psychological thriller that was on BBC 1 last year, it's basically about a detective in London hunting some pretty horrifying killers. You know the murderer from the beginning and it's about how they catch him, but also the psychological element of how the characters feel when they confront the murderer and know he is guilty but cannot convict him etc.. There is also a great sub-plot involving Luther and another character that I don't want to ruin, but I will say it's a very unique and surprising story. I'm not doing a great job explaining it but I promise it's a great show and definitely worth a watch, here's a trailer anyways (incidentally the one that got me to watch it originally)