Fresh from the imperial examinations, Duan Ling volunteers to journey to the northern city of Ye, where the empire's border is under threat of a Mongol invasion. Accompanying him is Wu Du, tasked with rebuilding the city's ragtag garrison into a force capable of holding Ye through the winter. When Duan Ling arrives, however, he discovers that the Mongol army is led by none other than his childhood friend, Borjigin Batu. The grown-up Batu is nothing like the boy Duan Ling remembers--and now, to prove himself to his own people, Batu must capture the very city Duan Ling has sworn to defend. While Duan Ling and Wu Du brace for Batu's attack, back in the capital, suspicions mount around the false crown prince. As the empire's most feared assassins vanish from the capital one after another, more than one person begins to wonder: What deadly secret could draw all Four Great Assassins to the north?