Analysis comes from the Cahill Command Center about the notes: they refer to a Johannes Kepler book recovered from the castle. The Cahills go to the book's current location at the Library of Philosophy and Cosmology in Prague but cannot enter without a reference. After going to Sedlec Ossuary on a hunch from the Command Center, they discover a Vesper One report on a flash drive and the initials AJT, Arthur Josiah Trent, carved on the wall, which shocks Dan because Arthur Josiah Trent was their father. Erasmus, a fellow Cahill, explains that Arthur was initiated into the Vespers but later cut off ties with them. After receiving an email from the library saying they can go, Dan and Amy enter the Library of Philosophy, they bump into Jake and Atticus, who help the siblings. Inside, Katja Mavel, the library director, gives them what they want. They find the map in the book. In Rome, a Vesper dressed like a waiter kills William McIntyre, who leaves a secret message in his shoe before he dies. Back in Prague, Amy and Dan tell Atticus what they are doing, and Atticus reveals his involvement as a Guardian, a separate family allied with the Cahills. Dan connects Il Milione and the map to Samarkand, Uzbekistan - their next destination. Vesper One asks them to drop off the package at the Astronomical Tower, near the statue of Jan Hus. They do, but the Vespers kidnap Atticus, who followed them. Vesper One now knows Dan and Amy have been hiding Marco Polo's epilogue and explains that Atticus was the price of keeping it from him. Minutes later, AJT sends another message to Dan's phone.
