Nocturne

July 1966, and while the country is gripped by World Cup fever, Oxford is doing its usual thing: pretending the summer is all punts and Pimm's. At the Museum of Natural History, a specialist in heraldry and genealogy called Adrian Weiss is murdered, almost in full view of a group of schoolgirls.

The girls are summer boarders, stuck at the nearby school because their families cannot take them on holiday. To pass the time they play pranks based on the story of the family that originally owned the building, a clan that made its fortune in the tea trade with India and was murdered at home a hundred years earlier. It starts as harmless mischief. Then a twelve-year-old girl is murdered at the school, and the old story stops being a game.

The case leads to a fifth-generation descendant of the family's head, a man who worked out that a change in the law on inheritance through an illegitimate child could line his pockets, and who was prepared to kill to make it happen. Morse reads it all out of Weiss's detailed genealogical files. Along the way a masonic ring connected to the case goes missing, because nothing in this show is ever straightforward.

Russell Lewis wrote it, Giuseppe Capotondi directed. Jack Laskey is DS Peter Jakes, Shaun Evans is Morse, and the schoolgirls are played by Susy Kane as Miss Victoria Danby, Imogen Gurney as Edwina Parrish, Nell Tiger Free as Bunty Glossop, Anya Taylor-Joy as Philippa Collins-Davidson, Eve Perry as Antonia Lockwood, Maya Gerber as Stephanie Hackett, Lucy Boynton as Petra Briers, and Emily Renée as Shelly Thengardi.

The pranks the girls play are genuinely unsettling because you know, the way the episode knows, that they are rehearsing something terrible. Anya Taylor-Joy and Lucy Boynton in the same school roll call is the kind of casting gift that later seasons would make a habit of.

It is historical detective work of the best kind, using the innocuous backdrop of summer mischief to get at generations of guilt and old money secrets.