As the chaos from the fierce sea battle at Sacheon subsides in the sweltering summer of 1592, Daimyo Wakisaka Yasuharu (Sung-kee Ahn) arrives in Busan to take control of the Japanese naval base with an iron grip. His objective is straightforward: to eliminate the threat of Joseon commander Yi Sun-sin (Hae-il Park), whose indomitable spirit and strategic mind have unsettled the Japanese forces. Wakisaka's initial mission involves gathering intelligence on the enemy, leading him to interview frightened survivors from the recent battle. They recount chilling tales of a formidable new Joseon battleship, labeled the "bokkaisen" (sea monster) and "mekurabune" ("blind ship"). Despite the dread they inspire, the discussions also reveal significant weaknesses in the design of this ship. Its inflexible dragon head can get stuck after violent ramming, and its flank armor is alarmingly vulnerable. Yet, determined to turn the tide, Wakisaka resolves to collaborate with the remaining Japanese navy in an effort to secure a decisive victory and subsequently set his sights on Ming China.
Simultaneously, Yi is gathering his finest admirals at the Left Jeolla Navy base in Yeosu for a critical meeting concerning their imminent naval actions. Won Gyun (Taecyeon), who leads the Western Fleet, passionately advocates for a defensive approach rather than being the first to strike. As news of the Japanese assault on Pyongyang spreads and the king's hasty retreat to Uiju becomes known, many commanders begin to acknowledge the merit of his argument. However, Yi remains undeterred and focused on crafting an innovative strategy, calling it "Crane Wing", a daring crescent-shaped formation to trap the enemy from multiple sides. His audacity earns him praise from Junsa (yohan-byuen), a samurai he had previously wounded during battle and who had since been taken captive at Sacheon. Just as Yi seems to be gaining traction, the clever maneuvering of Japanese spies leads to chaos as they infiltrate his base, liberating their comrades, sabotaging the turtle ships, and stealing essential construction documents. In a hasty reaction, Yi travels to Suncheon to consult with Na Dae-yong (the inventor of the vessel), realizing the imperative to conceal his turtle ships from Wakisaka until their design can truly shine.
While Wakisaka scrambles to understand Yi's intricate battle plans, his aggressiveness is further amplified by reinforcements led by Katō Yoshiaki, his fierce rival, who arrives with superior ships and artillery along with Kuki Yoshitaka. They formulate a vigorous two-pronged attack targeting the Left Jeolla Navy headquarters with their full force.
Yet, unbeknownst to Wakisaka, Yi's skilled operatives—Lim Jun-young and Jeong Bo-reum—have successfully infiltrated his ranks and are feeding him vital information. As suspicions rise, Wakisaka decides to sever ties with Katō and Kuki, seizing command of their fleets and resources in a bold move to assert his dominance. Additionally, Yi remains alert, bracing himself for a trap that he suspects will be laid by Wakisaka at the significant Gyeonnaeryang Strait. Formerly aligned with Wakisaka, Junsa discreetly shares critical details with Yi, alerting him about the imminent ambush.
Despite Junsa’s warning reaching Jeonju—the fortress that shields the land access to the naval base—his pleas for caution are ignored as Wakisaka launches a surprise attack. Junsa finds himself captured by Joseon’s Righteous Army, where he encounters Hwang Bak, the militia's commander, who, while skeptical, decides to unite forces with Junsa.
Cunningly, Yi responds to Wakisaka's advance by drawing him into the open waters around Hansan Island. When Wakisaka's fleet approaches the strait, the commanders Eo Yeong-dam and Lee Un-ryong serve as bait, though they are almost too powerful for Wakisaka to manage. Unwavering in purpose, Yi deploys the upgraded turtle ships as Na advised, awaiting the opportune moment to initiate his assault.
As the battle ensues, the negligence of Won Gyun provides a window for Wakisaka's troops to invade the Joseon lines. Yet, Yi quickly counters using the improved turtle ships, merging the flanks of Eo and Lee as they culminate the Crane Wing formation. Cornered and outmaneuvered, Wakisaka faces a devastating defeat, with his forces shattered and himself left injured and afloat.
In his final, desperate bid, as Wakisaka attempts to ram Yi's flagship, an arrow from Yi finds its target, leaving the Japanese commander reeling. Back on land, the Righteous Army, rallying around Hwang Bak, successfully defends the passage, even at the cost of their commander's life.
This hard-won victory emboldens Yi as he continues his assault, striking at Angolpo and Busan, ultimately compelling the Japanese to halt further naval engagements. As he withdraws to his base, Yi reflects on the formidable achievements in his campaign, knowing that his most remarkable triumph - the establishment of a new base on Hansan Island within the year ahead - stands to showcase the resilience of his unwavering resolve.
