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?

With new information about Chief Constable Ziegler, John is sent back to the police station to get closer to DI Russell Carter, his brother's new partner who mysteriously arrived around the same time that James disappeared.

A new case takes John and Russell on a walking tour of Cambridge, where they are faced with an impossible question: why would someone kill a tour guide that they'd only just met? To get closer to the truth about what happened to conspiracy blogger Roger Sinclair, Lucy goes undercover and makes a shocking discovery of her own.