The mysterious journal of Jacobite exile Mary Dundas has lain unread for 300 years. Amateur codebreaker Sara Thomas is hired by a historian to crack the journal<U+2019>s cipher, but when she arrives in Paris, Sara finds herself dealing with complications from all sides: the journal<U+2019>s reclusive owner, her charming neighbor, and Mary, whose diary doesn't tell the secrets Sara is expecting to find.