Mac has injured his back saving a young girl's life and has been told that he will need to spend the next six weeks in bed. In desperation he asks the police for some work to keep his mind off his increased pain levels. He receives 23 cold case files, all brutal unsolved murders, and Mac gets to work. All he can do is apply his mind but that is more than enough. With help from the mercurial DS Kate Grimsson, a new recruit, and DC Tommy Nugent Mac re-opens the case of an axe murder carried out by a drugs gang. However there is a lot more to the case than initially meets the eye.But there is another cold case that Mac just can't put out of his mind. He gives it his best efforts but will he ever find out what really happened?