Puzzle setter John ‘Ludwig' Taylor's life of solitude is upended when his identical twin, DCI James Taylor, disappears. Drafted in by his sister-in-law Lucy, John reluctantly assumes his brother's identity as a detective in the Cambridge Police Authority in order to find clues to his brother's whereabouts.

John is tasked with infiltrating the Major Investigations Team, gathering information and solving a completely unconnected murder, all whilst remaining in character as his brother. A difficult ask for anyone, let alone a reclusive puzzle-setting genius who rarely leaves the house. Can this master of all things cryptic crack his biggest puzzle yet?