When a woman vanishes from Baker Street station, the Police investigation leads nowhere until her kidnapper leaves the first of a series of coded messages. Unravelling the clues challenges the ingenuity and intelligence of the Metropolitan Police's most skilled officers. Their task is made far harder when they realise that they are not the only ones looking, and finding her first is a matter of national security. It becomes clear that she is a woman with much to hide and much to hide from.