Rose Hudson is a mathematician, puzzle-solver, and former cryptographer for the British government. After becoming a widow, she moves to the United States in 1979 and buys a house in the beautiful Berkshire Hills of western Massachusetts to start a new life. She strikes up a friendship with the town's chief of police, Richard Prince, which leads her to help solve the baffling death of a famous violinist. Richard questions suspects while Rose uses her code breaker skills to assist him and, despite her hesitations, a romance develops between them. When Rose discovers an unsavory event in the violinist's past and uncovers the reason for his murder, both her newfound happiness and life are in grave danger.