Puzzle solver John "Ludwig" Taylor's life of solitude is upended when his identical twin, DCI James Taylor, disappears; he reluctantly assumes the identity of his brother as a detective in the Cambridge Police Authority to find his whereabouts.

After a breakthrough finding a coded notebook, John decides to return to the police station to gather information about his missing brother's last case; Lucy embarks on a fact-finding mission to the home of one of her husband's ex-colleagues.

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

A body falls from the fourth story of a construction site; questions arise if it is just a terrible accident, or a perfectly executed murder; when things don't add up, John questions his abilities.

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

With Lucy embroiled in a murder plot, all eyes are on John; the hunt for James takes a back seat as John desperately battles to prove Lucy's innocence; angry at James for abandoning his family, John questions whether his brother is worth any of this.