Puzzle setter John 'Ludwig' Taylor's life is upended when his identical twin, DCI James Taylor, disappears. John reluctantly assumes his identity in order to track him down.

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.

A body falls from the fourth storey of a construction site. Was it just a terrible accident, or a perfectly executed cuddle? When things don't add up, John questions his abilities.

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

A cryptic late night call from Holly sees Lucy lured into danger. Back at the station, all eyes are on John as the pressure ramps up. Will his cover finally be blown?