On a mission to crack the code in his twin’s notebook, John returns to the police station. A frustrated Lucy takes matters into her own hands and plays detective herself.

A new case takes Ludwig on a walking tour of Cambridge, where he’s faced with an impossible question. Meanwhile, Lucy goes undercover and makes a grim discovery of her own.

John and the gang go back to school to investigate the suspicious death of a headmaster. Lucy sneaks off on a covert mission, and a shocking revelation leaves John reeling.