In 1259, the Mongol army laid siege to the Southern Song stronghold of Diaoyu Fortress, and the situation was extremely critical. Detective Thunder Pass received a commission from a friend to follow a grain-supply boat, delivering food and top-secret firearm blueprints to the fortress. However, secret intelligence revealed that a Mongol spy had infiltrated the accompanying team...If anything went wrong with the Mongol army [at Diaoyu], it would inevitably affect the operations of another Mongol force that was then attacking the capital city of Damascus in the Middle East.