Betrayal Behind Monastic Walls. Finally vacation! But what are Oskar's parents planning? Archaeological excavations at Maulbronn Monastery. Oskar finds it really boring - and quickly disappears into the Middle Ages to join his friend Albrecht Dürer. However, he also ends up in a monastery here - and in an exciting mystery involving a valuable icon, false pilgrims, and betrayal behind monastic walls. 'Oskar and the Secret of the Monastery' is the third time travel adventure that Oskar experiences in the Middle Ages, following the first two books 'Oskar and the Secret of the Missing Children' and 'Oskar and the Secret of the Children's Gang.'.