On the eve of the Rebellion of the Seven States in the Western Han dynasty, Cai Yun, the only son of the Cai family, is recruited into the army as a spy for a certain prince. To protect the Cai family's sole heir, their household guardian deity, Mu Yan, a 2,800-year-old snake demon, takes human form and accompanies Cai Yun into danger. With Mu Yan's help, they survive the many crises that arise within the prince's army. But fate has other plans: both men fall in love with Huan Nuoyun, the second daughter of the prince's general. Later, when the Rebellion of the Seven States breaks out, the Battle of Dantu separates the three. Taking advantage of the opportunity, Mu Yan takes Huan Nuoyun to live near the Xiongnu. Cai Yun, meanwhile, returns to the Crown Prince's side after the war... As luck would have it, Cai Yun is framed by a villain, thrown into the imperial prison, and sentenced to execution after the autumn harvest. The news eventually reaches Mu Yan. As the Cai family's guardian deity, what will he choose? And how will the tangled love and hate among the three come to an end?
