Sgt. Truman (Stanley Baker), a policeman who deals with juvenile delinquents, visits the Murphy household after a theft by the young twins, Mary and Patrick. There he meets their older sister, Cathie (Anne Heywood), and soon begins a romantic involvement with her. However, Cathie's brother, Johnnie (David McCallum), is the leader of an infamous local gang, and doesn't like to see his sister with a cop. Gradually, Truman starts to see a link between Johnnie and a string of local arsons.