After the Hungarian army was annihilated by the Mongols at the battle of Mohi, only the castle of Esztergom stood in the Mongolians' way of invading the rest of Europe. Batu Khan, the grandson of Genghis Khan, reaches the walls of Esztergom with his invincible troops. The castle's defenders, led by Eusebius, the canon of Esztergom, and a Spanish mercenary, Captain Simon, are preparing for the final battle. The sudden arrival of Cardinal Cesareani, the Papal Legate, coinciding with the Mongol Lunar New Year celebrations and the mystical practices of Eusebius, combine to offer the defenders a small glimmer of hope.