While rummaging around Grandma Pearl's attic, sisters Tracy Doyle and Amy Allen discover a box of old letters written to their grandmother in 1945 from a woman named Peggy living in Washington, D.C. Each letter has a reference to a Bible verse in the Book of Numbers. When they also find an old Bible with cryptic notations in the margins next to those exact same verses, the sisters' curiosity is piqued. With a little digging, Tracy concludes that Peggy was a code breaker during WWII. But was their beloved grandmother involved in secret war work, too? And if the historic puzzle isn't intriguing enough, their cousin Robin confides that someone is stealing valuable items from her antiques shop. Tracy, who promises to discover the thief, complies a list of suspects who have opportunity and financial motives. But when one of the items mysteriously reappears on its shelf, Tracy wonders if the unknown culprit is stealing for some other reason altogether.