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.