After all the investigations Sister Philomena's conducted this summer, she needs one. In this adventure, Sister Philomena, her niece, Delaney, and nephew, Riley, are invited to Siena, Italy for the famous Palio horse race and festival. Their vacation plans abruptly change when they stumble upon mysterious documents while visiting the childhood home of St. Catherine of Siena. It's possible they have found a centuries-old letter written by Saint Catherine along with a ransom note addressed to the Pope. Of course they must investigate!

Their search for answers takes them on a fast trip across Europe to Avignon, France, and the famous Palace of the Popes. In the course of their investigation, the children learn about Saint Catherine and the history of the Church in the 1300s when the papacy was forced to flee from Rome to Avignon for safety.