Old, Unhappy, Far Off Things (S05E01)

The final reunion celebration at Oxford's last all-female college is underway, honoring a beloved professor who's about to retire. However, the festivities take a dark turn when a guest is discovered dead. Lewis suspects the murder is linked to a violent incident that occurred in the same location a decade earlier. He reaches out to his former detective sergeant, Alison McLennan, for assistance in unraveling the mystery, but the investigation takes a shocking turn when two additional victims are found.