Years ago, the kingdom of Pei suffered a significant loss when the vital city of Jingzhou fell into the hands of the kingdom of Yang. This defeat was largely due to the infamous duel between the popular and talented Commander Ziyu and the seemingly unbeatable Yang Cang. Incensed by Ziyu's reckless challenge to Yang Cang for a rematch without his approval, the King of Pei demands that Ziyu and his wife, Xiao Ai, perform on the guzheng, a traditional Chinese plucked string instrument. Rather than give in, Ziyu defiantly cuts off his hair as a sign of his refusal before he is permitted to leave the court.
To secure peace between the two kingdoms, the King decides to betroth his cherished sister, Qingping, to Yang Cang's son, Yang Ping. However, a shocking revelation comes to light: the Commander Ziyu that everyone knows is not the real Ziyu. The real Ziyu lies severely injured from their previous encounter, hidden away in a cave as he recovers. The man in his place, revealed to be Jingzhou, was trained as a "Shadow" by Ziyu's uncle due to his striking resemblance to the Commander. Only Ziyu and Xiao Ai are privy to this life-altering secret, and Jingzhou takes on Ziyu's identity in the aftermath of the duel. Ziyu devises a plan to retake Jingzhou, promising Jingzhou he will return to his home to live with his mother once they succeed.
The King seeks to punish the impersonator "Ziyu," demoting him to the status of a commoner, despite protests from military leaders. In a tense moment before his departure, the King examines the wound that "Ziyu" received from Yang Cang's sword and inquires about its condition. The distinctiveness of the cut suggests it was made recently, to serve as a reminder of his previous defeat. An envoy relays that Yang Ping has reduced the proposal to taking Qingping only as a concubine, much to the court's disgust and the King's acceptance. General Tian publicly denounces the King’s decision as humiliating and resigns in disgrace.
Meanwhile, Jingzhou struggles in training with his unique metal umbrella weapon against Ziyu, who emulates Yang Cang's aggressive techniques. Tensions rise as Ziyu’s frustration leads to harsh critiques, prompting Xiao Ai to suggest that Jingzhou should embrace his weapon's symbolism of femininity to counter Yang Cang's masculine spearplay, rooted in the philosophy of yin and yang. She even showcases the efficacy of this technique herself. To bolster their chances, Jingzhou recruits Tian to lead an army of convicts, whom Ziyu previously enlisted, and introduces him to the strategy involving Jingzhou.
Ziyu unveils his tactical plans to Tian, explaining he intends to utilize Jingzhou to reclaim Jingzhou by keeping Yang Cang distracted for three rounds. On the eve of their duel, Jingzhou confides a secret to Xiao Ai about his unwavering devotion towards her, culminating in an intimate encounter that night—one watched by Ziyu through a discreet peephole.
As the duel approaches, Jingzhou arrives by boat at Jingzhou City Pass, ready for battle on a floating platform. Hidden beneath the surface, Tian and the armed convicts launch a surprise attack. After managing to enter the city unnoticed, Jingzhou engages in the duel, claiming victory in the first round but ultimately succumbing in the subsequent rounds. Yang Cang proposes a draw, but Jingzhou, seeing the Yang banner still standing, challenges Yang Cang to finish to the death.
Tian and the convicts, wielding their innovative umbrella weapons, push back Yang Ping and his forces, though the conflict entails heavy casualties on both sides. Yang Ping confronts Qingping in a brutal duel, mortally wounding her. When he questions her about her motives for fighting, she manages to kill him using the dagger meant for their wedding. In the midst of the chaos, Tian manages to topple the Yang banner, igniting Yang Cang's fury as he proceeds to demolish Jingzhou in combat. Just as Yang Cang prepares to deliver a fatal blow, Jingzhou utilizes his newly learned technique, ultimately finishing Yang with the snapping tip of the weapon.
In a heart-wrenching turn of events, Jingzhou returns to his childhood home only to discover his mother brutally murdered. Just as he is cornered by assassins, a mysterious emissary appears to save him, proclaiming to represent the King. Returning to the capital of Pei, Jingzhou finds himself embroiled in yet another attempt on Ziyu’s life as more assassins infiltrate his former residence.
During a celebratory feast, the King makes a sudden declaration, asking all but Jingzhou and Xiao Ai to leave, indicating he wishes to declare them a genuine couple to reward them both. He announces a pivotal change concerning the command in the kingdom. Suddenly, a masked assassin appears, shockingly bringing forth a box assumed to contain Ziyu's head. When the King opens it only to find it empty, the situation escalates as the assassin unexpectedly stabs the King before revealing himself to be Ziyu. Enraged, Ziyu commands Jingzhou to kill the King, accusing him of orchestrating his mother's murder. In a tense confrontation, Jingzhou manages to tragically wound Ziyu and flips Ziyu's earlier scenario on its head. Ultimately, he uses Ziyu's weapon to slay the King, planting the sword in Ziyu’s hand as a clever ruse.
As Jingzhou exits the hall to faces the stunned officials, he proclaims that the King has been slain and that he was responsible for the assassin's demise. To his surprise, Tian appears unconcerned with his account of events. The film concludes with Xiao Ai racing towards the hall doors, overwhelmed and uncertain as she grapples with whether to reveal Jingzhou's actions or accept the precarious new circumstances.
