S5·E1 Old, Unhappy, Far Off Things

Oxford's last all-female college is holding a reunion party to say goodbye to one of its most popular professors. After a guest is found murdered, Lewis believes the killing is connected to an attack that happened at the same building 10 years ago. He turns to his former DS, Alison McLennan, to help him with the case, but it ends up taking a new twist after two more bodies turn up.