The story unfolds in the year 1598, marking the seventh year of the brutal Japanese invasion of Joseon. The tide of war has shifted following the death of Toyotomi Hideyoshi, who passed away after issuing the command for his forces to withdraw to their homeland. This comes just one year after the Japanese saw their final opportunity for a total conquest thwarted by the strategic brilliance of Admiral Yi Sun-sin. While the Ming Dynasty has allied itself with the Joseon forces to repel the invaders, the Japanese general Konishi Yukinaga finds himself trapped and besieged at Suncheon by Admiral Yi.
Driven by a desperate urgency to return home—fearing that his political rival, Tokugawa Ieyasu, is plotting to seize power in the wake of Hideyoshi's death—Konishi attempts to secure a safe exit. He tries to bribe various Ming admirals to grant his troops passage. After several failed attempts, he finds a weak point in Jae-yeong Jeong, who plays the role of Chen Lin. Susceptible to the bribes and the promise that no harm will come to his men, Chen Lin agrees to the clandestine request, betraying the alliance.
The tragedy deepens for the Joseon forces when Yi's youngest son, Yeo Jin-gu as Lee Myeon, is murdered by the Japanese as a cruel act of retaliation for the victory at Myeongnyang. Despite this devastating personal loss, Yi remains steadfast and determined to see every single invader annihilated. His suspicions are aroused when reports surface of Japanese warships entering Chen Lin's camp without any resistance. Alongside the Ming vice admiral Deng Zilong, with whom he has formed a genuine bond, Yi confronts the traitorous admiral. Upon discovering evidence of the collaboration, Yi threatens to dissolve the entire alliance unless Chen returns to his duties. His unwavering resolve earns him the grudging respect of Jae-yeong Jeong, and the admiral eventually rejoins the combined fleet.
Seeking to eliminate the primary threat to their retreat, Konishi manipulates fleet commander Shimazu Yoshihiro by claiming that Yi intends to use the retreat as a pretext to launch an assault on Japan itself. Shimazu agrees to help Konishi by eliminating the Admiral. Anticipating a swift move, Yi decides to intercept Shimazu's fleet at the Noryang Strait. Under the cover of darkness, Yi sets sail, leaving his Japanese retainer Junsa behind to maintain a facade of a blockade, deceiving Konishi into believing the fleet has not moved.
The ensuing battle is fierce. Yi and the Ming fleet successfully trap Shimazu in a pincer movement, but the tide turns when Shimazu deploys captured Joseon cannons, effectively evening the odds. As the conflict escalates at Suncheon, Konishi realizes the deception and rushes to aid the relief force. In a moment of extreme peril, Yi asks Chen and Deng to hold the line against Shimazu while he engages Konishi. The Ming ships are nearly overwhelmed; Deng is killed in the chaos, and Chen is on the brink of defeat. A small force led by Junsa intervenes to save them, though it costs Junsa his life.
As Shimazu concentrates his full power on Yi, the Admiral stands atop his flagship, beating the war drum to rally his exhausted allies. In the heat of the battle, Yi is fatally wounded by an arquebus shot. Despite the mortal blow, he refuses to falter, maintaining his position until he finally collapses. In his final moments, he issues one last order:
> Keep my death a secret until after the battle is over.
Following his father's wish, Yi's son Hoe dons the Admiral's armor and takes his place at the drum, masking the tragedy. Re-energized by the sight of their leader, the allied fleet launches a final, crushing charge, driving the demoralized Japanese forces back and inflicting crippling losses.
Once the victory is secured and the truth is revealed, the entire fleet assembles to mourn the fallen legend. Yi is given a hero's funeral befitting his sacrifice. His surviving commanders deliver the news of his passing to Crown Prince Gwang-hae, who honors the eternal inspiration and legacy that Yi Sun-sin gifted to the people of Joseon.
