This is one of a handful of current British police procedural programs that I think of as elite, nearly perfect. (Think Versa, think DCI Banks, think Shetland.) I don’t mean literally perfect: every show disappoints at times or drops the ball briefly, but you don’t notice or care. The show is a slow burner, for obvious reasons. There are no serial killers on the loose, no hostage situations or thrilling car chases. The cases Cassie’s team investigates are old. They are hindered by the fact that before they can question witnesses, research possible suspects or persons of interest, they need t...