After 91-year-old Amelia Hoffman dies in her flat on a busy London road, detectives Arthur Bryant and John May of the Peculiar Crimes Unit discover she was once a government security expert, though no one can quite remember her. A link emerges between the old lady and a diplomat trying to flee the country, and it seems that an impossible murder has been committed. In this 18th case (after Oranges and Lemons), Bryant is convinced that other forgotten women with hidden talents are also in danger—and, curiously, they all own models of London Bridge.